@Hilary259 Check to see that you are typing in the correct URL.
If you think the URL is correct, see if typing it into another device like a laptop or desktop computer returns the same 400 result. If so, the URL is definitely wrong.
If typing the URL into another device works well, and you are able to view and edit your listing, verifying your login ID and Password, you should be able to safely delete the APP from your iPad and reinstall it, which should solve your issue.