Restart your phone and see if the Camera app is still Disabled in the Settings >> Apps menu.
If it is and you still can't Enable it, try restarting your phone into its Safe Mode.
https://devicehelp.boostmobile.com/scenario/Max_XL/Boost_Mobile/en/Activate_Safe_Mode
If the camera app is working normally again while in Safe Mode that indicates there's some app you've installed that creating this odd conflict. Can you think of anything you installed around the same time the Camera app stopped working?
Another thing to try is to wipe the system cache partition on your phone. The system cache is different and separate from app caches, and it's not something accessible through the Settings menu, you need to restart your phone into its Recovery Mode:
https://www.hardreset.info/devices/zte/zte-boost-max/recovery-mode/
Once in Recovery Mode you'll be in text-only user interface so use the indicated buttons to navigate through the various options and select 'wipe cache partition'. They system cache doesn't contain anything vital nor does it have anything to do with your saved files so it won't adversely affect anything. But it's worth trying in the event something like a corrupt file in the system cache is preventing the camera app from functioning.
A last resort option is to do a Factory Reset but be sure to do a full back up of your data and files. Or a less destructive, workaround solution would be to install a third-party camera app and use that instead. I highly recommend 'Open Camera'. Be sure to delve into the app's Settings menu after you insta it to take advantage of its very expansive feature set options. iar