Hi Tinen,
We have checked your account and see that you have subscribed for the Adobe PDF Pack service.
PDF Pack is an online subscription that can not be used through the Adobe Acrobat DC desktop application. If you have installed the Adobe Acrobat DC app then and trying to use your subscription through it, then it wont work.
PDF Pack you can access online via- https://cloud.acrobat.com/exportpdf or can installed the free desktop application Adobe Acrobat Reader DC- Adobe Acrobat Reader DC Install for all versions
*I assume you have used the trial version of Acrobat DC which has now been ended and its asking for serial number. You can uninstall the existing version of Acrobat DC using the- Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs so that you would not get the serial number prompt again.
Hope this helps.
Regards,
Akanchha