A MAS file, just in case you are not aware is a compressed file with the track resources in it. It is looking for a specific file to load those resources and can't find it.
Open the scn file for the track with notepad, at the top will be pointers to the MAS files it needs, and the paths where it is expecting to find them. Example, chosen at random:
//-------------------------------------------------------
SearchPath=.
SearchPath=MontTremblant09
SearchPath=MontTremblant09\MontTremblantGP09
//-------------------------------------------------------
MASFile=MontTremblant.MAS
MASFile=MontTremblantMaps.MAS
MASFile=commonmaps.mas
//-------------------------------------------------------