By default, DSSR checked for ATOM/HETATM records with the alternative location (altloc) set either as space (most common) or 'A'. In PDB entry 6CEV, the DNA atoms have altloc as 'B'. That's why you do not see any results from a default DSSR run.
Try: x3dna-dssr -i=6cev.pdb --altloc=B
Best regards,
Xiang-Jun