There is some misunderstanding in terminology. I use "covariate" as a continuous variable in the context of linear mixed-effects modeling, not a variable of no interest. So change the following 2 lines in model.txt to:
Model:Income+Alcohol+Marijuana+Gender*Exposure (or Income+Alcohol+Marijuana+Gender+Exposure)
COV:Income*Alcohol*Marijuana
you should be fine.
Gang