Activation problem with new imac

HI there.
I tried to activate parallels desktop 15 to a new imac. In my account I have 2 licences. One for Parallels desktop 13 and one for 15.
During the activation proccess I enter my serial and I get the message "You have entered an upgrade key. Please confirm it with a key from the previous version.". I proceed by entering my previous licence key for paralells desktop 13 but the activation fails by saying that the servers dont recognise the key.
Please provide some help. I suppose that a reset is needed in my active keys.
Thx in advance for your assistance.