ExportPDF does not have any kind of activation key, serial number or anything like that. What is asking you for one?
It asking me to buy again each time I try to convert a file.
Everything is based on your Adobe ID.
0. Make sure you are not trying to export to Powerpoint.
1. Sign out from Acrobat Reader. Sign in again with the Adobe ID used to purchase.
2. If still not working, go to http://www.adobe.com/ and sign in with the same Adobe ID; check your account details. Make sure ExportPDF is shown as an active subscription.
3. If still not working try using https://cloud.acrobat.com/exportpdf