AWS group user not allowed to assume role - access denied

I have a user and I'm trying to impersonate a role for running a service on Kubernetes. However, when I tried using STS to assume the role, I get the following error:

$ aws sts assume-role --role-arn "arn:aws:iam::{ACCOUNT_ID}:role/service-myservice" --role-session-name AWSCLI-Session

An error occurred (AccessDenied) when calling the AssumeRole operation: User: arn:aws:iam::{ACCOUNT_ID}:user/me is not authorized to perform: sts:AssumeRole on resource: arn:aws:iam::{ACCOUNT_ID}:role/service-myservice

I find this odd because this user belongs to a user group with the AdministratorAccess permission attached to it, which should give it access to anything on AWS. This is it:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "*",
            "Resource": "*"
        }
    ]
}

So, what am I doing wrong here?

What you have here is the IAM policy attached to this User, aka - what is this user is able to do.

You need to set the Trust Relationship as well. This defines which resources or principals is able to use this role/user. Could be Lambda, EC2 or in your case: an IAM User.

See here for example.

The IAM User/Role (in that case, role) you want to assume must have the Trust Relationship as follow:

{
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
          "AWS": "arn:aws:iam::<account_id>:role/<role_name>
        },
        "Action": "sts:AssumeRole"
      }
    ]
  }