What do I do if my iPad mini says apps not compatible?

The iPad mini 1st generation cannot update past iOS 9.3.5 or iOS 9.3.6 for a cellular model..

It has a 32-Bit processor and 512 MB RAM

Your old iPad mini can only run 32-Bit apps.

FYI: All new apps are now 64-Bit.

It's time to buy a new or Apple refurbished iPad.

Your question says "iPad mini" but your profile line says "iPad 2, iOS 9".

What system are you running on your iPad mini, and what model/year is it?

I should warn you that as an iPad gets older, eventually it can no longer run the current system due to increased resource requirements, then eventually app publishers stop supporting the older system software and hardware. As a result, the number of compatible apps available gets smaller and smaller.