Help with line on headboard of bed

I need help please. I followed this Tutorial on meshing a bed in blender for Sims 4. I did very well with the tutorial and the follow up tutorials, but once I import the mesh and texture to S4S there is a diagonal line visible on the headboard. Where did I go wrong? How can I fix this problem?

I would appreciate it if someone could help me and advise me on how to get rid of that line pretty please.

Kind regards