MEE6 cannot give/remove roles or kick and ban bad people when his role is too low in the hierarchy.
There's a simple solution to that, here's the recipe.
Check that MEE6 role is high enough in the hierarchy
Go to your Discord server settings, then click on Roles. Then, click on MEE6's role and move MEE6's role above the other roles.
On mobile, the interface will look different, but you need to do the same — move MEE6's role above the other roles.
You will also need to move your Custom Bot role higher, as shown below, if you are using one.
If MEE6 is still showing you the error message, please join our support server. We'll be happy to help.