Unable to resize images in photo apps on Mac Catalina

Yer_Man is exactly right-- try a restart of the Mac. If that doesn't work, try a restart 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 

Do you use any "Cleaner" apps or "Virus Killers?" Those are known to cause crazy problems.

Let us know how it goes-- we might have other ideas…

When you select Edit Mode, can you edit the picture at all, or is it just Crop that doesn't work? Can you not go into Crop mode, or do you get an error message? Does the Crop screen look like this, more or less?

(I say "more or less" because this is a Tahoe screen, not Catalina.)

What happens, exactly?

This could be a problem with the Library or with the User account. To find our, try these things:

• 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.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to temporarily move your Library out of your own user's Pictures folder up to the general "Users" folder. 
• 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.

Have you tried restarting the machine? That often fixes odd glitches like this.

suefrompapamoa wrote: … Hi, i can edit photos ok.This is what it looks like.

You have the little handles to drag into a crop-- what happens? Does it just not drag?

léonie may be right that's it's the OS, so if you can upgrade, you should. But if not, I'd try switching Libraries and switching users like I suggested earlier.

Did this happen directly after upgrading to Catalina? Are you using iCloud Photos.

Is it possible to upgrade to a more recent system version? I remember, that I also had problems with cropping after the Catalina upgrade.

What do you mean by "resize"? Sorry to answer a question with another one, but on here I've seen folks use that word to describe

• cropping an image (which is removing parts of the photograph)
• Scaling and image (leaving the photo the same but reducing its dimensions)
• changing the file size, the kb or MB of the actual file itself

So can you clarify?