Why does my Mac Mini say Closing the library? I am trying to open PICTURES

I assume you are using the new Photos app. The first time or so that you use Photos it has a lot of work to do to finish creating the Photos library. I had similar problem I just let the library close on it own overnight and the next day I could open and close Photos with not problem.

To get rid of this, you need to hold down your power button until your screen goes completely black - don't worry this is only resetting it - when turned black press the power button again and your laptop will come back on. Then you sign in to your laptop and the 'closing library' should have gone. This worked for me first time and I haven't seen the closing library again.