Hi. Your mesh is too high poly to be imported in s4s. It is made of 160k tris, while Maxis most complex meshes almost never go past 10k tris. 60k+ can crash the game for many people, ~130k is s4s limit
You need to return to MD, delete stitches ( they alone account for 120k tris), undo Remesh, quadrangulate the mesh, increase particle distance dramatically until the polycount is 10k at most
If you want the garment to have fold and stitches, you must make high poly and low poly versions, then
bake textures from the high poly to the low poly one. These details are not game assets friendly
Unrelated to this problem, there are other issues:
- the tutorial you watched was very unfortunate, everything needs to be redone. You should disregard everything they told you to do or never do ( "never delete the body", etc);
- instead of your current reference, it is best to choose a lingerie set or a swimsuit. Same overall shape, same functionality, but less sculpted details ( all these hems lead to a ripped result of uv_1 transfer ). But if your garment had a skirt, you would need to choose a dress of similar length, not a swimsuit ( again, with as little sculpted details as possible );
- do not join the body with the garment until the latter received weights, uv_1 and vertex paint. The body is already adapted to perfection and overwriting this data can mess it up;
- transfer all data with nearest face interpolated, otherwise your mesh will move and morph like crumpled paper;
- all areas of the body covered by the garment must be deleted, the uvs of the garment should go in the freed space;
- do not scale uvs of the garment individually unless the difference in quality is acceptable. It makes sense for buttons to be of better quality then the rest, but parts of the garment made of the same fabric should not differ in quality;
- this garment must be vertex painted with 00FF00, not a random green color. There are other hex codes for different areas/categories, so it is always best to just transfer vertex paint instead
