If u know how to solve this please tell me, have got this once before and i solved it but dont remember and i think its different each time but if you know please tell me.

I think I solved it clicking on Create UV's > Planar Mapping and selecting the whole model. I don't know what it does but I don't get the error.

yes... this error is related with UV's...  in most cases i had that error is more then one "UV set" in mesh .... just copy sets to 1st one normally its called "map1" then remove rest...