Hi!
Our portal dynamically populates this drop-down. Once you select a Region and a Key Pair, we send an API request to AWS inquiring what machine types are available and put them in the drop-down.
This messages implies that for some reason, the AWS API is returning no results. I think the best thing you could do is possibly contact AWS and ask them why via API there aren't any machine types being return.
Out of curiosity, if you select a different region, do you get machine types?