Screenshot

You’re getting tight on space, but not to the point of being unable to save screenshots.

A forced-restart of your iPad might clear-up any latent issues or memory management problems.

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On other iPad models: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

As to your installed version of iOS, your iPad is correctly running the most recent version. A first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.1.

Apple ended major update support for these models of iPad in September 2019 - although they since continue to received “point” security updates. These devices cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (CPU and RAM) for newer versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with your iPad, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS14.

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?

You can find all this info in the Settings app, General settings panel, About.

Actually, LotusPilot is incorrect.

Your iPad year, model and storage size is almost full!

16 GBs storage iPad starts out, after initial drive formatting, at approx. 14.5 GBs of free, internal storage.

After the installation of iOS 12 at approx. 5.5 GB, leaves approx. 9 GBs of of free, internal data storage space.

iOS/iPadOS needs a minimum of 2-4 GBs of constant remaining storage, that MUST BE MAINTAINED AT ALL TIMES, to insure/ensure nominal, acceptable operation of all iOS/iPadOS functionalities.

This leaves a 16 GBs iPad with ONLY a practical, usable storage space of between 5 and 7 GBs AND THAT IS IT!

Your iPad has 6.3 GBs left meaning, you have less that 1.0GB of actual, usable internal data storage space left to use.

This is virtually NO internal data storage to do much of anything with that iPad!

This is why Apple quietly, silently did away with all 16 GBs iOS device back sometime in 2016.

Your iPad is starved for internal data storage space, with so little internal data storage space to begin with.

Your info doesn’t help me.

I suspect your iPad has run out of available data storage space.

6.3 GBs out of what is that total amount of available internal data storage space of your iPad?

Again, this is found in the About section in the Settings app.

It is the only reason I can think of why the OP is not able to take screenshots, any longer.

I realise that screenshots are only a few MBs, but iOS/iPadOS need internal data operational room which is sorely lacking on the Poster's iPad.

Then you and I can agree to disagree on this point.

To repeat - the OP is getting tight on space, but not to the point of being unable to save individual screenshots (measured in a few MB).