“User has been created, but could not be added to this site”

Could be a plugin/theme conflict. Easiest way to check for that is to install this plugin and follow its instructions on how to check for a plugin/theme conflict:
https://wordpress.org/plugins/health-check/

As that plugin was created by the WordPress community for situations like this. It allows you to deactivate all plugins and switch to a default theme without it affecting anyone else visiting/viewing the site.

https://make.wordpress.org/support/handbook/appendix/troubleshooting-using-the-health-check/