Hello!

Activation Codes are obtainable from Special Events here in Forums. Aside from that, the only way to get Activation Code is to use a browser to log in and claim the "Activate the Flash". Thank you.

I took the code from "Activate the Flash" but it doesn't work?
I have never claimed nor redeemed any codes since I am new to the game. I have no other past accounts.

Whenever I enter the CORRECT CODE, a popup that says "Exchange failed, serial number is invalid." shows up on my screen. ( I have double checked the code and it still gives me an error.)

Is there anything I can do to fix this?

That is because it expired most likely, also that code wasn't even worth the effort as it just gave you garbage items you don't need.