Roblox shirt upload keeps failing - says to use the template

if the size really is exact and it still gets rejected, re-export from a different editor. some save a 16-bit png or embed a colour profile and the uploader doesn't like it. a plain 8-bit png with transparency has never failed for me.

If it's already 585x559, check that the exported file really is that size. Right click it, Properties, Details. Some editors save at a different size than the canvas or add a pixel margin, and the uploader rejects anything that isn't exact. Also make sure you picked Shirt and not T-Shirt as the type. T-Shirts are just a square decal, so a full template uploaded there fails with that same message. You don't need to remove the background.