Hi & welcome to the Forums.
The 20th Anniversary sale does indeed include Sonic 2, when you made the purchase you would have received an email from Sony containing a receipt for your purchase, before contacting them about this I would try re-downloading the bundle pack, if that doesn't work then i suggest contacting Sony using the details in the email you received.
Customer Service - 1-800-345-SONY (7669)
Account & billing Support -1-877-971-SONY (7669)
Available Mon to Fri 8am - 8pm PST