I have Apache webserver (the one supplied by Apple) running on my Macbook and it ran flawlessly, until I upgraded to Catalina 10.15. I used the same httpd.conf, but now it is di
i had the same error, the problem in my case was that macOS Catalina comes with the php module disable by default, in the error log apear as:
tail /private/var/log/apache2/error_log
[Wed Feb 26 18:10:57.123036 2020] [autoindex:error] [pid 3145] [client 127.0.0.1:53734] AH01276: Cannot serve directory /Library/WebServer/Documents/subdir/: No matching DirectoryIndex (index.html) found, and server-generated directory index forbidden by Options directive
the apache server was loking for index.html instead of index.php so it was blocking the list directory, I uncommented the line #LoadModule php7_module libexec/apache2/libphp7.so in /etc/apache2/httpd.conf and restart the apache server
sudo apachectl restart
and it just work. hope it helps.