@lapiito Have you resetted your wallet?
By this i mean close your wallet (check in the tray next to your windows clock!), then reopen it, login in the online wallet and try again to plot... It should work!
If you are logged in in your local wallet and it has not completed downloaded the blockchain, provably to your local wallet your burst address is not activated yet, because it has not downloaded until the block where you activated your account... ;D