A strange thing happened today.
I wanted to manage plugins for one of my WordPress websites.
But, I could not see any add/deactivate/delete plugin options on the Plugins page.

Then I realised that I had WordPress Multi-Sites enabled as I installed it that way earlier.
So, only Network Admin can do that.
Just go to Network Admin -> Plugins from top left and you will see Add new, Network Activate/Deactivate options there on that Plugins page.

Cheers!