When you go to fetch your Seller ID and MWS Authorisation Token, you will see the following screen:
Try to ensure that you:
Correctly copy and paste the Seller ID AND NOT the Marketplace ID.
Correctly copy and paste the MWS Authoritisation Token alphanumeric identifier beginning with "amzn.mws."
Be sure to copy/paste and not write it down manually. When writing and you are more likely to make a mistake.
If none of the above helps, be sure to visit Amazon MWS Support (https://sellercentral.amazon.co.uk/gp/mws/contactus.html?ref=ag_xx_shel_xx) to see if there is a problem with your Amazon MWS Permissions Manager.