Return this iPad and get your money back!
iPads stuck on some version of iOS 12 will be completely phased out by sometime in 2022.
All iPad and iPhone models stuck on iOS 12, all third party app support will end by the 2022.
This downtrend of third party app support has, already, started this year and will continue until 2022, or so.
iOS 12 devices are, already, 7 and 8-years old in 2021.
If you need OR ONLY able to purchase older, less expensive iPad models, look at older IPads from 2014 and later.
All of these older model iPads below CAN run the latest releases of iPadOS 15.
2014, 9,7 inch screen iPad Air 2 models
2015, 7.9 inch screen iPad Mini 4 models
2015-2017, 12.9 inch screen iPad Pro models. ( 1st gen Pencil support )
2016, 9.7 inch screen iPad Pro models ( 1st gen Pencil support )
2017, 10.5 inch screen iPad Pro models ( 1st gen Pencil support )
2017, 9.7 inch screen iPad 5th gen models
2018, 9.7 inch screen iPad 6th gen models ( 1st gen Pencil support )
2019, 10.2 inch screen iPad 7th gen models ( 1st gen Pencil support )
2020, 10.2 inch screen iPad 8th gen models. ( 1st gen Pencil support )
Get the largest internal data storage model you can afford as NO Apple iOS device has ANY expandable storage.
I am sorry that you had to learn all of this, but this does not change any of the facts regarding these older iOS devices.
Also, iPad screens are all measured on the diagonal of the actual screen area.
So any case/cover you purchase for any iPad uses this diagonal screen dimension.
Sorry & Best of Luck to You!