Screen won’t lock after upgrading to Catalina
Check the battery settings in the Energy Saver. You should be able to see a dial to change the time needed for your Mac to sleep. The defaults for these have been known to change from time to time, so you may be able to put it back to what it was originally.
Incidentally, locking your computer by leaving it unattended for 10 minutes is not a good security practice. If you want a quick way to lock it, see here for sleeping and here for locking. This is a habit that is worth picking up.