A FRCSYM2 error means that the program tries to construct a little cube of density around an NCS-related point to a point in the mask (in order to do the interpolation), but that it fails to do so. The remedy is to take a map which either contains at least an asymmetric unit plus one or two points on all sides (e.g., with EXTEND in the CCP4 suite), or to take a whole unit cell, or (in simple cases like dimers) to take a map which encompasses the reference molecule (around which the mask is defined) and the NCS-related one(s).