"Cannot generate ST coordinates" errors occur when your UV map has a triangles or quads with co-linear or coincident points in the UV or 3D (it might only be UV, but your model shouldn't have colinear/coincident points in either 3D or UV because if nothing else it's a waste of polygons that will never display).

Arma needs to be able to convert from any point on your UV map to a unique point on your 3D faces and vice versa. If all your UV points for a particular face are on the exact same point, or all 3-4 points fall on a straight line, then it's ambiguous which 3D coordinates correspond to the UV point.
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