https://www.google.com/search?q=plain+shirt https://www.amazon.com/s?k=plain+shirt on edit: https://www.google.com/search?q=shirt+-stripes
Ah, but you see, he specifically wants a shirt without stripes as dialectically opposed to a shirt with stripes. A plain shirt just won't do. See: https://www.youtube.com/watch?v=wmJVsaxoQSw
"Shirt without stripes" is harder to imagine it meaning "strips on the side. I'll add them myself".
Language is ever so complicated and contextual.