I strongly recommend you use a newer version of Modeller. 6v2 is very old.
DELETE_RESTRAINT RESIDUE_IDS='123’ ‘124'
DELETE_RESTRAINT takes ATOM_IDS, not RESIDUE_IDS. For example, you might say
DELETE_RESTRAINT ATOM_IDS = 'O:56' 'C:56' 'N:57' 'CA:57'
to remove any restraint on the given dihedral.