
So, you're not able to do this, right?
Try the following in order as needed:
1 - close your library and reopen it holding down the Option+Command keys to restore/repair the library. Test again.
2 - create a new test library and import a Live Photo. Try to edit it. Then close the test library, open the primary library and test again.
3 - boot into Safe Mode from the external SSD according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.
Note: 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 assure a successful boot into Safe Mode.
