Why my admin tool plugin suddenly missing on moodle 3.8, i don't know why. The folder file still available on file server but why missing on web?
Help me please,

Why my admin tool plugin suddenly missing on moodle 3.8, i don't know why. The folder file still available on file server but why missing on web?
Help me please,
I can't do anything without this admin tools plugin, such an uploading user the plugin didn't show up
Check the file permissions on the admin/tools directory - it looks like the web server is unable to read from there.
The only reason why you should ever change $CFG->admin to anything other than 'admin' is if your hosting provider has reserved [yoursite]/admin for some sort of control panel (and I'd suggest that, if that is the case, then they may well not be a great set up for any other aspect of running Moodle).
If you really do need to change $CFG->admin, then yes, you need to rename the admin folder in the code to match (but, again, please don't do that, unless there is a really good reason).
See: https://github.com/moodle/moodle/blob/master/config-dist.php#L168
With Moodle 3.8 and 3.9 there's an issue if you change $CFG->admin: MDL-69061. I don't think the site would work correctly.