Make sure you include AWS.Credentials when you create the AWS.SageMakerRuntime instance. So for example:
var creds = new AWS.Credentials({
    accessKeyId: 'your-string', 
    secretAccessKey: 'your-string'
});
var sagemakerruntime = new AWS.SageMakerRuntime({
    region: 'us-east-2', 
    credentials: creds
});
If it's not that, then I'm not sure what's the issue here.