Cannot open Photos app due to 'User is changing the system photo library' error

Instead of opening Photos, try double clicking on the Photos Library. The Photos Library is in the Pictures folder.

If that doesn't work, try

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Go to Photos' Settings>General

"Use as System Photos Library" should be grayed out. Make sure the path to the Library makes sense, and be sure that "Copy Items" is checked.

• If those don't work, rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.

I've never seen this error, but probably one of these will work. Let us know…

If all else fails try launching your library with the Option+Command keys held down to repair/restore the library.

tmb wrote: …Restarting the computer solved the problem.

Great! I'm so glad it was easy!