İpad 5 th lock unlock problem

Do you mean when your iPad Mini 5 is in this case, presumably one which when you open the case cover your iPad should open and when you close it should lock? If that is what you mean, is this case specifically designed for iPad 5 Mini?

In Settings > Display & Brightness > Is there a Lock/Unlock button?

Typically, if the case the correct case for the device, once the iPad recognizes the case, the Lock/Unlock button appears and you turn it on for it to work with the case.

Did you buy an authentic Apple case? If not, does the manufacturer for the case you purchased suggest it is compatible with Lock/Unlock? If it wasn't designed for Lock/Unlock, there is nothing you can do except manually open and lock the iPad when you open or close the case.

You're welcome.