Title: WordPress ‘Add New Plugin’ not showing.

Hello, this is my first time posting on the forum. I could really use some help.

When I logged onto my website today I found that the ‘Add New’ button on the plugins page was missing. The last time I logged in to my website (about a month ago) everything was working. I also only seem to be able to deactivate and activate plugins instead of being able to delete them. I deleted all the plugins (using my hosting company’s file manager since that was the only way I could) along with all the themes, resetting to the default Twenty-Twenty-One theme. I also noticed that the updates page was giving me updates but when I went to the page in question it didn’t say it needed updating.

An example:

I have two websites which I am currently controlling. There is a plugin which they both have in common (All-in-One WP Migration) The website which is not affected has it updated currently to 7.45 and is telling me to upgrade to 7.46

But when I login to my other website, it is on 7.43 and is not telling me to upgrade at all. The thing is, when I go to the updates page it tells me to update, but not on the plugins page. The same things happens with the themes.

The other thing is – when I logged in today – I got a plugin error which said something along the lines of ‘ninja-file-manager/filester: not found’. I disregarded this as a pointless error until I saw the button was missing.
The weird thing is I’d never even heard of Ninja File Manager until I looked it up after this encounter and I did not have it installed.

FYI, before I uninstalled all the plugins and themes, I backed up my website so if anything happened I could restore it.

I installed Health Check using the file manager and it gave me these two errors.

– The DISALLOW_FILE_MODS constant is defined and enabled.
– A plugin has prevented updates by disabling wp_version_check()

I looked this up in the forum and it said that there would be a line (DISALLOW_FILE_MODS) in my wp-config.php file. When I looked inside the file I couldn’t find that line or any line that was similar.
As far as the 2nd error, I didn’t have any plugins installed except the health check when I got that error so I’m not sure what the problem is there.

Here’s some general info about the website that I’m experiencing the problem on. I don’t know if you might find it useful.

Hosting Provider: Hostinger UK (https://www.hostinger.co.uk/)
Hosting Plan: Premium (100GB Storage, 100 Websites)
Domain Registrar: Namecheap
Server Type: LiteSpeed
WordPress Version: WordPress 5.8
Theme: Essentials Theme (https://themeforest.net/item/essentials-multipurpose-wordpress-theme/27889640)

Do you know what could be the issue?
Any help would be much appreciated