Thank you for posting on Adobe forums.
Could you please confirm what Adobe product or online service have you subscribed for. Is it Adobe ExportPDF online service?
Are you able to sign in at "https://cloud.acrobat.com/exportpdf" using your Adobe ID credentials or using Adobe Reader (what version) for the same?
Please share the screenshot of the exact error message that appears so that I can analyze this issue.
Hope to get your response.
Please make sure that you're signing in to the Reader app for iOS with the same Adobe ID/email and password that you used when you signed up for the ExportPDF service.
If you continue to run into problems with the subscription, please contact Apple (as Adobe doesn't manage subscriptions purchased through the Apple App Store).
[topic moved to Reader/Acrobat DC for Mobile forum]