Hey @Edpa57, normally if you would purchase on the Google Store, the access-code will be sent out 2-3 days after shipping.
You mentioned though that you bought Stadia from someone on Facebook. Therefore that person should have gotten (or will get) the access-code and has to give it to you. Otherwise you would be unable to play Stadia. Therefore I would recommend to try to get in contact again with that person to get the code. If this is not possible, you should probably cancel the purchase... the only other thing you can do is: If you have received the hardware (serial number on it) and that guy can also give you the order-number with Google, you might contact the Stadia support to receive a code from them. But without an order-number of Google it's not possible.