If you are trying to turn off Stolen Device Protection when Face ID is not working, there are two options to try.
First, when you are prompted with Face ID, if there is a Try Again button, keep tapping that and failing Face ID until you are prompted with entering your passcode. Thus turning off Stolen Device Protection.
Second, if there is no Try Again button, you have to log onto iCloud.com/find on the web then select your iPhone from the list and Remove From Account. That doesn’t require Face ID and will turn off Activation Lock (Stolen Device Protection). Then you can turn Find My back on after and Stolen Device Protection will remain turned off.
Note that there may be a 1 hour security delay when turning Stolen Device Protection off.