In most cases this means you have another account on the Hub (possibly one registered using Facebook/Apple ID and one via email) where you have already linked to this Strava account.
Log into any other accounts and unlink Strava and try again. If you are not aware of any other Hub accounts please contact us with your Strava ID number (available from your Strava home page link link i.e. https://www.strava.com/athletes/1234567) and we will look into this for you.