I think this bug actually belongs with Apple, but a quick Google shows that people have been asking about it in the Apple forum with no resolution for a while, so I thought I'd reference it here in case you have more leverage.
After a sequence of editing steps, Apple Photos (in MacOS Catalina, latest update) will no longer allow a photo to be further edited and shows a dialog 'Cannot Start Editing - Photos cannot load adjustments for this image.". Edit mode does actually launch, but the tools are not visible and the ... menu won't allow 3rd party tools to launch.
It's taken me a while to be able to reproduce the error reliably and perhaps there's a simpler way, but these are the ones I can do this 100% with.
1. Select a 'Live Photo' image.
2. Click Edit
3. Choose the Crop tool and make an adjustment
4. Click Done
5. Click Edit again
6. Click the ... menu and choose Pixelmator Pro
7. At the 'Turn off Live Photo', say OK
8. Make a change - I've been doing a repair to clone out a spot
9. Save Changes
10. From the dialog, choose 'Save Flattened'.
11. Click Done
12. Click Edit yet again
13. Make a change, say to the color or exposure
14. Click Done.
15. Click Edit once more and ... Boom.
I realise there's a lot of moving between the editor and the normal view, but that happens.
Pixelmator Pro is in this equation because I think the repair tool is the best of all the software I have, truly amazing.
Once the error state has been reached, the only option is to use the 'Image -> Revert to Original' menu path and start again.
Is something not working like it should? Let us know.
And I just remembered, I posted about this here back in April. I should have added this to that thread, so perhaps an Admin can move it for me?