Intro to design appears to be unsolvable.

[JiuJitsuJunkie]

There is no arrangement or amino acid choice that will allow successful completion of the task

[rmoretti]

As hinted by the text, you should be able to get a better score by reducing the size of the two red amino acids.

There should be several combinations which give a passing score, but the "simple" approach of reducing the size by mutating both to Alanine - ALA(A) - should meet the requirement. It's not the best combination, but it should pass the threshold. (While Glycine - GLY(G) - is even smaller than Alanine there are energetic reasons related to backbone structure why it's not a good choice in this situation.)