Error with FVM permissions when installing new website

The entire WP tree should be owned by the same user under which PHP is running. If you have that, then the error you see should go away.

Are you using PHP-FPM on your VPS? If so, what user do you have set for the pool which runs this set? And what user owns the WP file tree?

Pardon the suggestion, as you may be using a VPS to learn about Linux, but if you don’t know these things, perhaps a VPS is not the right solution for you. You might do better with either “normal”, shared hosting or a managed WordPress plan on a more powerful platform.

Where is your VPS hosted? What flavor of Linux are you using?

PHP usually runs as the user wwww-data on Debian, so make sure the wordpress directory tree is chown’d to www-data:www-data.

Note that this isn’t the best way to do things securely, as each website on your VPS should run under a different user, just as each would use a different database with a different user.

I really, really suggest you consider that you’re in a hole and stop digging. Your former dev did you no favor. Switch to some other plan at OVH or another host so you’re not the system administrator of the server. If you’d like to go back to that someday, get a cheap vps ($5/month at a place like Digital Ocean) and learn enough Linux to make that happen. Don’t hold your website hostage to your level of Linux knowledge.

It concerns me that you say the only user on your site is root. Logging in frequently as root is a bad idea as it’s really, really (really!) easy to shoot yourself in both feet. You should be using an unprivileged user and using “sudo” whenever you need to do rooty things.