Hi,
Are you using the variables in the selection or a RKF ?
Are u doing it in Dev or PRD ?
Check the Properties of the Variable and RKF..
Something Strange...Check out once again..if you are able to Offset Debit then you should be able to do it for Credit aswell.
Alternate Solution.
Also copy the already offset Debit and rename it to Credit after changing the KF to Credit.
ALso I guess You have not save the Credit RKF with the Variable and trying to Create a Copy of it..
rgds
SVU