Hello and welcome to Apple Support Communities UncleBrian_,
We see that you're unable to use Markup on a screenshot that you've taken on your iPad. We understand wanting to be able to use this feature of your iPad to make adjustments or add on to a screenshot, and we'd be happy to help.
First, let's have you restart your iPad, and also make sure that you're using the latest version (14.4) of iPadOS on your iPad.
Update your iPhone, iPad, or iPod touch
If you are already using the latest version, and the issue remains, after restarting, take a look at the articles below with some additional information that can help.
Draw in apps with Markup on iPad
Add typed text, shapes, and signatures with Markup on iPad
If the issue persists, can you tell us if this happens with all screenshots that you take? Or does this only happen when taking screenshots in a specific app or apps? Have you been able to take screenshots and use Markup in the past without an issue?
We look forward to hearing back from you.
Have a wonderful day!