I selected security updates and other updates in update manager and I clicked 'install updates'. It showed up an error like so 'Requires to install untrusted updates' and I clicked 'close', and it was the only button visible. The update manager started to build the list of updates again and it showed up 'install updates' button once again. Same thing happened again and again and I was not able to install any updates.
In the software centre go to edit and software sources (type in your password) one of your software sources in the other sources tab does not have a key.
This does not necessarily mean it is bad but that you would need a key
Check this url:
This will help you to resolve the error.
You can install Muon package manager and use it instead of the update manger, its great to install it write on you terminal: