How are you trying to use it and what is happening? Generally this is due to the user trying to log in with a different user name/password than they used to sign up but that's not always the case.
Try signing out, then sign back in again with your Adobe ID and password.
We're sorry for the trouble you had while accessing the services of Adobe ExportPDF, the record shows that you have an active subscription to Adobe ExportPDF on your current Adobe ID.
Try accessing the service through the URL https://cloud.acrobat.com/exportpdf with your Adobe ID and password on a browser like Chrome, or Firefox, or Safari.
If you are using freeware Adobe Reader and facing any issue, sign out and sign back in Adobe Reader and then check for any pending updates of Adobe Reader from 'help>check for updates', reboot the machine after updating Adobe Reader.
You can also refer to the Adobe article for more troubleshooting Troubleshooting Create PDF, Export PDF, Adobe Send, Fill & Sign … services(PDFPack or Document Cloud...
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