Fields inactive mode in substitution screen

Hi Team,

i am trying to write substitution in OBBH, i entered company code, calling poing & double click onsubstitution. it enter to inside. here i am trying to write a step some field in BSEG table are inactive mode. it is giving the message of "You cannot select the element here"

You cannot select the element here

Message no. SFB507

Diagnosis

You have attempted to select an operand that is not allowed, from a syntax viewpoint, at this point. Only operands that are shown highlighted may be selected at this point.

Procedure

Select an appropriate operand by double-clicking on it.

how to activate

regards

Srin