FVerbaas posted at 4:37 PM Fri, 6 May 2022 - #4438209

Does your shirt already have a uv map? If you have the .obj you can easily check whether there are vt entries. 

Next possibility it has a uv map but the coordinates are not limited between 0 and 1. That is what uv mapper wants.

UVMapper will give a UV coordinates out of range error for UDIM.

Easiest way to UV map anything organically shaped is a UV functional software that let's you mark seams, pin verts, stitch, normalize shell texture sizes, and ideally relax or smooth the UV shells. Judging from OPs comments he's trying to use the free version of UVMapper which has very limited functionality for UVing complex forms, just a guess. Planar, cylinder, box unwrapping usually will have serious regional distortions... planar along the side edges of the seams of a typical shirt pattern. Perhaps ok for a start unwrap but requiring further cutting, stitching, and tweaking for sure. Blender and Wings are free and have most of the map editing features mentioned. As much as I used to like Wings for modeling and UVing, latest releases leave alot to be desired. If I were looking to pick up a UVing skill with future proofing I would try to learn the process in Blender, IMO.