There seems to be an intermittent issue with the website. If you have been redirected to the purchase page even though having the active subscription, then you may check the following.
1- Please ensure that you are using the correct application Adobe Reader DC not Adobe Acrobat DC. There is a possibility that you have used the trial for Acrobat and that isn't working with the current subscription.
2- If you have Acrobat DC installed, please uninstall it using cleaner tool- http://labs.adobe.com/downloads/acrobatcleaner.html and install Adobe Reader- Adobe Acrobat Reader DC Install for all versions
3- Check online through https://cloud.acrobat.com/ Sign Out and Sign In once.
Let us know if it still doesn't work.