Giovanni,
-bsVars 'Int_Cov+NoInt_Cov' \
What kind of variables are Int_Cov and NoInt_Cov? Are they truly between-individual quantitative variables are declared above? If they vary within individuals, they should be treated as within-individual variables. In that case, you should use 3dLMEr:
3dLMEr ... \
-model 'Session*Int_Cov+Session*NoInt_Cov+(1+Int_Cov+NoInt_Cov|Subj)' \
-mask simulation_mask+tlrc'[0]' \
-qVars "Int_Cov,NoInt_Cov" \
-gltCode SessionIntCov 'Session : 1*A -1*B IntCov: ' \
...
Check out the discussion here for the general usage of 3dLMEr.
Gang Chen