importing Photos doesn't work

In many cases it is the file system format of the external drive.

macOS Sonoma 14 is more restrictive with respect to external drives than the previous system versions. If the library is on an external drive with a file system format that is not supported by Photos, where the library could get damaged, Photos will not allow us to import more photos to this library.

The format of the external drive needs to be macOS extended (Journaled) or APFS. If your external is formatted ExFAT, for example, it will no longer work. Make sure , your external drive has been prepared as described here: Move your Photos library to save space on your Mac - Apple Support

Move the library to a different drive, if the external drive is not supported, then format the drive with Disk Utility. Formatting will erase it, so you will have to save your data temporarily somewhere else.

Okay, it's time to try these things:

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

           Start up your Mac in safe mode - Apple Support  

• Make a new user and see if the same problem recurs with it
• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Andreafilox wrote: ... I tried to put some pictures in a folder to import them in the new library and the same happened. I really don't understand why importing doesn't start.

I'll try with an another library located in the computer. Do you think could work?

By " the new library" do you mean that you created a new empty Library in your Pictures folder? If you're still using the non-compatible formatted drive, then all bets are off. Definitely try using a new empty Library in the Pictures folder.

Great, that you could fix it Andrea!