Substitution at Call point 3

Hi All

I want to substitute the G/L account in reference key 3 field at call point 3.

But when i try to create at callpoint 3, System is giving error message " No fields can be substituted at this point".

But as per SAP note we can release certain fields for call point 3.

Kindly help me to proceed further in this issue.

Regards

R.Karthigayan