LISREL Adding an error covariance between independent variables

#1
Guys, I am making a path analysis with only manifest variables.

After running the analysis, modification indexes suggest to add an error covariance between two independent variables. How can I do this?
I've tried modifying the SIMPLIS using "SET THE ERROR COVARIANCE BETWEEN VAR1 AND VAR2 FREE", but this doesn't seem to work.
Thanks