It can get frustrating when MEE6 is missing some permissions. Especially when it comes to giving/removing roles. Here's a solution that should help you out!
Check that MEE6 actually has the manage roles permission
Go to your Discord server settings then click on Roles. Then, click on MEE6's role and ensure that it has the Manage Roles permission.
If MEE6 is still showing you the error message, please join our support server. We'll be happy to help.