As per the description above, you are getting Purchase/Buy again message while using the Adobe Export PDF services, Is that correct?
You are subscribed to Adobe ExportPDF services on your Adobe ID, you can use the services through your online account, Sign in to your account at https://cloud.acrobat.com/exportpdf with your Adobe ID and password.
It seems that you have Acrobat installed, and due to which you are getting the message.
Please install the free desktop version of Adobe Reader to use the services of Adobe ExportPDF. More about Adobe ExportPDF: Using Adobe Export PDF Link: https://helpx.adobe.com/document-cloud/help/using-exportpdf.html
Make sure you are logged in with your Adobe ID and password in Adobe Reader.
Hope this helps, and keep us posted with the results.