I was offered 20% off my Federal return for completing my taxes thru Turbo Tax but the discount wasn't there when I paid. What now?

We won't know anything specific about your email and discount offer here in the user forum.   And nothing account-specific can be handled here, but I'll tell you a Support option below.

           Did you get that email recently or some time in the past?  I know some 20% offers in the past had a deadline to file (or pay) to get the discount.   

          At other times, during promos the TurboTax price is already reduced to include a discount, then tells people to file by a deadline and get 20%,   That makes people think they were supposed to get another 20% discount.

          Did your offer have a special link to a promo page at which to begin TurboTax?   Or did it say to begin at the usual TurboTax website?

In any case, your best bet at this point is to phone TurboTax Support and discuss your email offer with them, so they can see what your offer was and why it wasn't applied.

Support hours are 5AM-9PM Pacific, every day this time of year.

FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632