Cannot add Plugins anymore

This typically means WordPress trying to connect to the update server in the background, but couldn’t. This could be due to a simple internet connection issue, or may be the server is offline. This connectivity issue, which usually resolves itself after a while.

If not, try contacting your internet/hosting provider.

Also, review this tutorial: http://www.wpbeginner.com/wp-tutorials/how-to-fix-secure-connection-error-in-wordpress/