The error message you are receiving normally occurs if the authorisation process for obtaining your token was interrupted or not completed in some manner.
You should have a link to try the process again. If you do not then simply go to 'eBay' and click on 'Settings'.
Once you have clicked on the required link you will be taken to eBay where you can log in. eBay should then present you with a 'Grant application access' page. Clicking on the 'I agree' button will generate your new token and return you Frooition.
When performing the process you should ensure that you do not navigate away from any page sent from eBay and if at all possible do not open multiple browser tabs when clicking on links.
If you can't resolve the issue by following the above steps you should contact eBay support here.
If you are still experiencing issues while renewing your token, you can visit this article to help you through the steps taken from Frooitions side.