Hello nuno197,
Let's use the steps below to boot your MacBook Pro into safe mode to see if you are able to sync your photos from there:
1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac.
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.
You can also find those steps in this resource: How to use safe mode on your Mac - Apple Support
It can take a few extra minutes to boot into safe mode. Your screen may flash or flicker while in safe mode. Those behaviors are expected. Testing in safe mode runs a check on your disk and removes some cached files, which may help with any discrepancies with your available storage. Once you've tested that behavior in safe mode, you can restart to leave safe mode.
Should you still be unable to sync your photos, the recommendations from this resource can also help with creating more available space: Free up storage space on your Mac - Apple Support
Take care.
