Not Letting Me Turn On Registration When I try to change my ACCOUNT settings at admin/config/people/accounts

The new state (visitors) doesn't stay checked at Who can register accounts. It reverts just back to admin.

Does anyone know why?

Note:  I had a look in settings and all the files around it (D8). But I cannot see anything similar.
The config variable might be hard-coded in your codebase.

You could look for the string:

$conf[ or $conf['user_register'] = 0 

For example, in settings.php or elsewhere.

The exact string may vary depending on your codebase, e.g quotes, spaces etc..

That would override the variable configuration setting in the UI.