If you get this error message in spite of being a guardian of the watch, you have most probably paired the watch with its own number and you are now logging in using your own number.
Note: The guardian's number has to be different from the watch's phone number.
You can read our article where we explain what to do when the watch and the guardian have the same number, or send your phone number and the IMEI number of the watch to https://support.xplora.com/hc/requests/new, so that we can check how the contacts have been configured.