In order to be able to view the activity (or course, ...) the account accessing the information needs to be granted permission to the information by the account that holds the information. If someone sent you a link to an activity, but they didn't grant the appropriate permission, you can't view it.
To resolve this, you may need to login to Garmin Connect to view the information. If you're already logged in, the sender needs to update the privacy settings for the data. There are two options here. First, they could add you as a 'connection' and then share the activity with their connections. The second option would be to share the activity with the public.
The privacy setting for courses and activities can be changed by clicking the lock icon in the button bar when looking at the activity or course details page.
