PHP7.1 ですか？

であれば、
http://concrete5.co.jp/blog/concrete57513-php71
を参考にして下さい。

ありがとうございます。
でもPHP5.6.3なんですよ。
何回かやってるんですが、同じ結果で．．

うむ。

その Internal Error のエラー文ってゲットできますか？

こんな感じです。

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, you@example.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.

Apacheのerrorは
[Mon Jun 26 21:51:24 2017] [error] [client ::1] FastCGI: comm with server "/Applications/MAMP/fcgi-bin/php5.6.30.fcgi" aborted: idle timeout (30 sec), referer:http://てtest.dev/index.php/install
[Mon Jun 26 21:51:24 2017] [error] [client ::1] FastCGI: incomplete headers (0 bytes) received from server "/Applications/MAMP/fcgi-bin/php5.6.30.fcgi", referer:http://test.dev/index.php/install

です。

うーん。ちょっと情報が少ないですね。

ちょっと時間が空いてしまいますが、
週刊 concrete5 の時に画面共有でトラブルシューティングってできます？

MAMP のバージョン 3.5.2、php 5.6.10ですが、インストールできました。
php のバージョンとかなんですかね？
ちなみに環境情報です。

# concrete5 Version
Core Version - 5.7.5.13
Version Installed - 5.7.5.13
Database Version - 20160615000000

# concrete5 Packages
None

# concrete5 Overrides
languages/cs_CZ/LC_MESSAGES/messages.mo, languages/cs_CZ/LC_MESSAGES, languages/cs_CZ, languages/da_DK/LC_MESSAGES/messages.mo, languages/da_DK/LC_MESSAGES, languages/da_DK, languages/de_DE/LC_MESSAGES/messages.mo, languages/de_DE/LC_MESSAGES, languages/de_DE, languages/el_GR/LC_MESSAGES/messages.mo, languages/el_GR/LC_MESSAGES, languages/el_GR, languages/en_GB/LC_MESSAGES/messages.mo, languages/en_GB/LC_MESSAGES, languages/en_GB, languages/es_ES/LC_MESSAGES/messages.mo, languages/es_ES/LC_MESSAGES, languages/es_ES, languages/es_PE/LC_MESSAGES/messages.mo, languages/es_PE/LC_MESSAGES, languages/es_PE, languages/es_PY/LC_MESSAGES/messages.mo, languages/es_PY/LC_MESSAGES, languages/es_PY, languages/fi_FI/LC_MESSAGES/messages.mo, languages/fi_FI/LC_MESSAGES, languages/fi_FI, languages/fr_FR/LC_MESSAGES/messages.mo, languages/fr_FR/LC_MESSAGES, languages/fr_FR, languages/it_IT/LC_MESSAGES/messages.mo, languages/it_IT/LC_MESSAGES, languages/it_IT, languages/ja_JP/LC_MESSAGES/messages.mo, languages/ja_JP/LC_MESSAGES, languages/ja_JP, languages/nb_NO/LC_MESSAGES/messages.mo, languages/nb_NO/LC_MESSAGES, languages/nb_NO, languages/nl_NL/LC_MESSAGES/messages.mo, languages/nl_NL/LC_MESSAGES, languages/nl_NL, languages/pl_PL/LC_MESSAGES/messages.mo, languages/pl_PL/LC_MESSAGES, languages/pl_PL, languages/pt_BR/LC_MESSAGES/messages.mo, languages/pt_BR/LC_MESSAGES, languages/pt_BR, languages/ru_RU/LC_MESSAGES/messages.mo, languages/ru_RU/LC_MESSAGES, languages/ru_RU, languages/sv_SE/LC_MESSAGES/messages.mo, languages/sv_SE/LC_MESSAGES, languages/sv_SE, languages/tr_TR/LC_MESSAGES/messages.mo, languages/tr_TR/LC_MESSAGES, languages/tr_TR, languages/cs_CZ/LC_MESSAGES/messages.mo, languages/cs_CZ/LC_MESSAGES, languages/cs_CZ, languages/da_DK/LC_MESSAGES/messages.mo, languages/da_DK/LC_MESSAGES, languages/da_DK, languages/de_DE/LC_MESSAGES/messages.mo, languages/de_DE/LC_MESSAGES, languages/de_DE, languages/el_GR/LC_MESSAGES/messages.mo, languages/el_GR/LC_MESSAGES, languages/el_GR, languages/en_GB/LC_MESSAGES/messages.mo, languages/en_GB/LC_MESSAGES, languages/en_GB, languages/es_ES/LC_MESSAGES/messages.mo, languages/es_ES/LC_MESSAGES, languages/es_ES, languages/es_PE/LC_MESSAGES/messages.mo, languages/es_PE/LC_MESSAGES, languages/es_PE, languages/es_PY/LC_MESSAGES/messages.mo, languages/es_PY/LC_MESSAGES, languages/es_PY, languages/fi_FI/LC_MESSAGES/messages.mo, languages/fi_FI/LC_MESSAGES, languages/fi_FI, languages/fr_FR/LC_MESSAGES/messages.mo, languages/fr_FR/LC_MESSAGES, languages/fr_FR, languages/it_IT/LC_MESSAGES/messages.mo, languages/it_IT/LC_MESSAGES, languages/it_IT, languages/ja_JP/LC_MESSAGES/messages.mo, languages/ja_JP/LC_MESSAGES, languages/ja_JP, languages/nb_NO/LC_MESSAGES/messages.mo, languages/nb_NO/LC_MESSAGES, languages/nb_NO, languages/nl_NL/LC_MESSAGES/messages.mo, languages/nl_NL/LC_MESSAGES, languages/nl_NL, languages/pl_PL/LC_MESSAGES/messages.mo, languages/pl_PL/LC_MESSAGES, languages/pl_PL, languages/pt_BR/LC_MESSAGES/messages.mo, languages/pt_BR/LC_MESSAGES, languages/pt_BR, languages/ru_RU/LC_MESSAGES/messages.mo, languages/ru_RU/LC_MESSAGES, languages/ru_RU, languages/sv_SE/LC_MESSAGES/messages.mo, languages/sv_SE/LC_MESSAGES, languages/sv_SE, languages/tr_TR/LC_MESSAGES/messages.mo, languages/tr_TR/LC_MESSAGES, languages/tr_TR

# concrete5 Cache Settings
Block Cache - On
Overrides Cache - On
Full Page Caching - Off
Full Page Cache Lifetime - Every 6 hours (default setting).

# Server Software
Apache/2.2.29 (Unix) mod_wsgi/3.5 Python/2.7.10 PHP/5.6.10 mod_ssl/2.2.29 OpenSSL/0.9.8y DAV/2 mod_fastcgi/2.4.6 mod_perl/2.0.9 Perl/v5.22.0

# Server API
apache2handler

# PHP Version
5.6.10

# PHP Extensions
apache2handler, bcmath, bz2, calendar, Core, ctype, curl, date, dom, ereg, exif, fileinfo, filter, ftp, gd, gettext, hash, iconv, imap, intl, json, ldap, libxml, mbstring, mcrypt, mysql, mysqli, mysqlnd, openssl, pcre, PDO, pdo_mysql, pdo_pgsql, pdo_sqlite, pgsql, Phar, posix, Reflection, session, SimpleXML, soap, sockets, SPL, sqlite3, standard, tokenizer, wddx, xml, xmlreader, xmlwriter, xsl, yaz, zip, zlib

# PHP Settings
max_execution_time - 30
log_errors_max_len - 1024
max_file_uploads - 20
max_input_nesting_level - 64
max_input_time - 60
max_input_vars - 1000
memory_limit - 256M
post_max_size - 32M
sql.safe_mode - Off
upload_max_filesize - 32M
ldap.max_links - Unlimited
mysql.max_links - Unlimited
mysql.max_persistent - Unlimited
mysqli.max_links - Unlimited
mysqli.max_persistent - Unlimited
pcre.backtrack_limit - 1000000
pcre.recursion_limit - 100000
pgsql.max_links - Unlimited
pgsql.max_persistent - Unlimited
session.cache_limiter - <i>no value</i>
session.gc_maxlifetime - 1440
soap.wsdl_cache_limit - 5

ずいぶん前の話ですが、新しいMAMPのバージョンで5.7.5や8でもインストールできない問題ですが。FastCgiServerのタイムアウトを変更すると問題なくなりました。

デフォルトのMAMP_FastCgiServer_MAMP
を
#MAMP_FastCgiServer_MAMP
でコメントアウトして
FastCgiServer /Applications/MAMP/fcgi-bin/php7.0.26.fcgi -idle-timeout 3600

php7.0.26.fcgのところは、お使いのバージョンでやると問題なくインストールできます。