This error is usually shown by FreeAgent for one of three reasons:
You haven't yet verified your email with FreeAgent
You need to complete the setup of your FreeAgent account (this is a set of questions you're asked when you log into your FreeAgent account for the first time)
You aren't set as the owner of the FreeAgent account you trying to connecting to
If you've completed the first two steps, you can confirm you're the account owner in your FreeAgent account. Just go to Settings and select 'Users'.
If you set up your FreeAgent account through your accountant, you may need to contact them for help with this.