If a collar says “Activation Failed” when you go to set it up, try it a few more times to make sure the connection is working.
If that does not work, you should try resetting the collar: Turning Off + Resetting the Fi Module
If you continue to have trouble, it’s possible that the collar is already assigned. Check the “Collars” tab under your main module and look for the serial number. If you do not see it, please email firstname.lastname@example.org with the serial number, so we can check if it is assigned elsewhere.