Moodle Installation Error #1 - Site is being upgraded, please retry later.
After certain steps in the installation process you might get this message. If you do, check your moodledata/localcache. I removed the .lastpurged file and it worked.
Moodle Installation Error #2 - Disable pcre.jit
The installation would ask you to disable pcre.jit. To do this, you must find your php.ini file on your server and add pcre.jit=0.