Can't define a new system library (Photos app)

Where is your current library stored, that you want to use as the System Photos Library?

The system Photos Library needs to be on a locally mounted volume with a wired connection (not a network share), if it is not in your Pictures folder or somewhere else in your user home folder.

If you are using an external drive to store the library, it needs to be prepared as described here:

Move your Photos library to save space on your Mac - Apple Support

It is essential that the file system format is MacOS Extended (Journaled) or APFS, and the external drive must not have been used for Time Machine backups or have any backup software installed.

Yohann_R wrote:

Surprisingly, there's no problem with the new user.

That indicates that there some 3rd party software active in your primary user account that's interfering. Therefore download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

What happens when you create a new Library (option-click Photos and choose "Create New") and add some pictures? Does that Library work? Do you have other Libraries on you computer?

What do you see when you sign in to iCloud.com and look at Photos there?

Have you tried making a new user and see if the same problem recurs with it?

One thing that you should try is to start in Safe Mode (different for different computers) This bypasses certain potentially disruptive processes.

           Start up your Mac in safe mode - Apple Support 

One of these steps may fix the problem, but their failure to work is also diagnostic.

Yohann_R wrote: Hi Richard, I tried safe mode to see if the problem was still there. And it's not!

First thing I would do is look in System Settings>General>Login Items and see what's there that's automatically happening just because you turned on you computer. If you see something that you don't have absolute confidence in, turn it off! Worst thing (and probably not) is that you later decide to turn it back on.

I'd do that even if your problem seems to be solved. Weird stuff creeps in...

steps to troubleshoot and potentially resolve the issue with your Photos library:

1. Restart Your Mac: Give your Mac a restart, as this often solves minor issues. After the restart, try opening Photos again.
2. Try Safe Mode: Restart your Mac while holding down the Shift key until you see the Apple logo. This disables third-party extensions that might be causing the problem. Afterward, restart your Mac normally.
3. Check iCloud Settings:

• • Click the Apple logo in the top-left corner of your screen.
  • Choose "System Preferences" > "Apple ID."
  • Ensure that "iCloud Photos" is turned on. If it's already on, toggle it off and then back on.

1. Use Terminal (Advanced):

• • Open Terminal from your Applications folder.
• Copy and paste this command, then press Enter:

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sudo mv ~/Pictures/Photos\ Library.photoslibrary ~/Pictures/Photos\ Library\ Backups/Photos\ Library.photoslibrary

• • This command moves your current Photos library to a backup folder. Try opening Photos again after running this.

1. Restore from Time Machine Backup:

• • If you have Time Machine set up, you can restore your Photos library to a previous state.
  • Open Time Machine from your Applications folder.
  • Navigate to a point in time when your Photos library worked fine.
  • Select your Photos library and click "Restore."

1. Contact Apple Support: If none of the above steps work, it might be a more complex issue. Reach out to Apple Support for professional assistance. They can guide you through more advanced solutions or repairs.

If the problem persists after you leave Safe Mode then run and post the Etrecheck report as requested.

It sounds like you're doing OK with Photos. Using iCloud is great to give you another way to see and share pictures. iCloud is a terrific system for synchronizing Photos on your devices. So your iPad, your iPhone, and your Mac can be used to view and even edit any picture, and you'll see the change on the other devices.

But be careful: iCloud Photos is not a complete way to backup your pictures. That's because if you delete a picture on your phone, it will also be deleted on all of your devices! Photos keeps them all the same. So most of us also have an external hard drive that we use to backup our Photos Libraries. I use Time Machine for frequent incremental backups, and I periodically copy the entire library to a separate external hard drive.

So don't neglect backups just because you got iCloud to work!