Cloud9 will manage IAM credentials automatically. This default configuration is currently not compatible with EKS authentication via IAM, we will need to disable this feature and use the IAM Role.
In the Cloud9 interface

In the Preferences interface

After turning off AWS managed temporary credentials
rm -vf ${HOME}/.aws/credentials

export ACCOUNT_ID=$(aws sts get-caller-identity --output text --query Account)
export AWS_REGION=$(curl -s 169.254.169.254/latest/dynamic/instance-identity/document | jq -r '.region')

test -n "$AWS_REGION" && echo AWS_REGION is "$AWS_REGION" || echo AWS_REGION is not set

echo "export ACCOUNT_ID=${ACCOUNT_ID}" | tee -a ~/.bash_profile
echo "export AWS_REGION=${AWS_REGION}" | tee -a ~/.bash_profile
aws configure set default.region ${AWS_REGION}
aws configure get default.region

aws sts get-caller-identity --query Arn | grep eks-blueprints-cdk-workshop-admin -q && echo "IAM role valid" || echo "IAM role NOT valid"

If the result is IAM role NOT valid, please check the previous steps to see if the IAM role information you created and assigned to Cloud9 Workspace is correct.