Error "Update to iOS 15.1 in order to download and use this application"

So, that in itself says little; as to Why..

Perhaps more random speculation &

total guesswork is required; to figure?

You should be able to tell us model/yr

and present iOS of your Device. It may

well be unable to upgrade to iOS 15.

..And should it present itself as road-end

iOS 12.x we'd, recommend a newer fone..

The hardware might be the cause of this.

Or, a "force-quit" could be an answer..

It was not supported my iphone6 because it will not download any apps