Problem scenario
In one region, but not another, you get this error with an "aws eks list-clusters" command:
"An error occurred (AccessDeniedException) when calling the ListClusters operation: Account 12345678910 is not authorized to use this service"
You know that IAM does not require region selection. What is causing this eks error?
Solution
Is your AWS CLI configured to use a region that eks does not support? To find out, try changing the region to us-east-1 (e.g., by running the "aws configure" command and responding to the prompts). If you change it to "us-east-1" and do not see the above error messages when you run the command(s) that created those messages, you should be ok.