kubernetes PodSecurityPolicy set to runAsNonRoot, container has runAsNonRoot and image has non-numeric user (appuser), cannot verify user is non-root

Here is the implementation of the verification:

case uid == nil && len(username) > 0:
    return fmt.Errorf("container has runAsNonRoot and image has non-numeric user (%s), cannot verify user is non-root", username)

And here is the validation call with the comment:

// Verify RunAsNonRoot. Non-root verification only supports numeric user.
if err := verifyRunAsNonRoot(pod, container, uid, username); err != nil {
    return nil, cleanupAction, err
}

As you can see, the only reason of that messages in your case is uid == nil. Based on the comment in the source code, we need to set a numeric user value.

So, for the user with UID=999 you can do it in your pod definition like that:

securityContext:
    runAsUser: 999