I came across this issue with one of my WordPress 5 installation. I was not able to update WordPress or find my Add Plugins menu item under the Plugins Menu.
So what do we do? On my search online, I found out that by adding the folloing line of code to your wp-config.php file, you could fix this quickly.
I am yet to find out what caused this issue in the first place, if someone of you gets to find it, I really like to know.