Why can’t I download and install OS 14 on this iPad mini 4?

Are you trying over the air or through iTUnes? If through iTunes try to do it over the air.

Hello Dahmjh1943,

Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are unable to update your iPad. To help with this, please see the steps in these Apple resources for your situation:

If your iPhone, iPad or iPod touch won’t update

If you can't update or restore your iPhone, iPad, or iPod touch

Cheers!