Oops Page Not Found Installing Uploaded Plugins

Hello,
Please can someone assist me..I have tried on and off for months to fix this.

I have my own VPS hosting and it seems out of the blue i am unable to install plugins or Themes via upload. If I FTP them up to the Plugins folder, They do not showing as un-activated plugins when I log into the WP-Admin–>Plugins section.

I can install from WordPress Marketplace directly.

To try to fix it , I have updated PHP and Mysql. I have plenty memory.

I even bought a new domain, and did a fresh WP install from cPanel on that and all I keep getting when I try to upload a plugin is: Oops! That page can’t be found. It seems to show the website front while it says that, even though the URL still says: http://www.myatvconnect.com/wp-admin/update.php?action=upload-plugin

It uploads, but on execution it flips to the website front and says Oops that page cannot be found.

I have check permissions tec.
Stumped.

Many thanks Glenn