iPad mini2 iOS 12.5.5 error, unable to update

This is true. The iPad mini2 does not have the hardware necessary to work with iOS 13 (or beyond). From time to time, you might see security updates to iOS 12, but there will be no more feature or performance updates.

What is the rated, internal data storage space/capacity of your iPad model and how much of this free, internal data storage capacity is left inside of your iPad model?

All of that additional info can, also, be found in the Settings app, General settings panel, About.

The 2013 iPad Air 1 ( “A” model number A1474/1475/1476 ), iPad Mini 2 ( “ A” model number A1489/1490/1491 ) and the 2014 iPad Mini 3 ( “A” model number A1599/1600/1601 ) cannot upgrade beyond any version of iOS 12, currently and very recently, at iOS 12.5.6.