To resolve this issue, you can try the following steps:
When connecting to Strava in JOIN, tap on the 'settings' button on the page where you are asked to grant permission to link your Strava account to JOIN. This will open Strava in a new browser window.
In the Strava window, make sure you are logged into the correct Strava account.
Return to JOIN and attempt to link your Strava account again, making sure to use the correct account this time.
Note that you do not need to create multiple accounts or change your email address to resolve this issue. Simply following these steps should allow you to successfully link your Strava account to JOIN.