So setting up this site, I was fighting with the error “Please provider a valid username” on the initial install. Stackoverflow etc pointed to just creating the wp-config by hand and moving on.
After following those guidelines, I then hit the error “Please provide a valid username.” when trying to complete install step 2.
After looking around, I decided just to add a phpinfo() call in the install.php to see what was going on.
Looking a the return data, I notice I’m not getting any $_POST data back, which wouldn’t help.
So a few more hunts, and I find the stupid issue that post_max_size was set to 25MB. For the php.ini it doesn’t like this, and even though it shows “correctly” in the phpinfo() output, this will cause the size to parse as 0 and block any $_POST data. Changing it to 25M fixed the issue.
Second issue that cause some fun was getting used to the FPM/FastCGI version of PHP, which requires a separate restart to get the php.ini file changes working.
For that you need systemctl restart php-fpm.service