The message shows up depending on the iPad model and iPadOS version you are using, based upon the way you may or may not have set up your security settings.
Here are a couple of articles that may help (or, at least, I hope not cause more confusion):
p.s. Welcome to the forum.
Card reader maybe - when you check in Files do you see something called "Untitled" at the bottom of the Locations sections on the left hand side of the screen?
(By the way I never get a message saying an accessory is connected when I attach my card reader to the iPad)
I have a different card reader for my laptop because the laptop is not a Mac. I have no difficulty opening photos on the laptop.
When I plug in the card reader, that I purchased for the iPad, I see a message that an accessory is connected. However, I am unable to find the card reader when I check in Files. Perhaps this is an issue with the card reader & not the SD card. Or maybe it is an issue with an older user that isn't very tech savvy.
I had hoped to be able to preview & sort photos while traveling and without packing my laptop.