I can open the app photos but I can’t open photos in the app

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

What happens when you try to open your Photos library?

Does it open?

Are the thumbnails of the photos visible?

What happens when you double click on a thumbnail?

You need to describe the problem in detail so we can get an idea of what you're experiencing.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

As a test launch Photos with the Option key held down and create a new, test library.  Import some photos and test to see if the same problem persists. Does it? This tells us if the problem is limited to your current library or is more wide spread.

Then close the test library and try your original library again.

If the problem persists create and log into a new, basic, admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists. This tells us if the problem is limited to your user account or is system wide.

Note:  do not launch any 3rd party apps in the account so as to not install any of the supporting files that most create.

If the problem isn't present then your original account has software installed that is most likely causing the problem.  

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Are you using iCloud Library and, if so, do you have "Optimize Mac Storage" enabled?

Where is your library located?