Waiting for answer.
Problem has been resolved.
The reason for this error is one hidden item in which value is moved using dynamic action from item filed. When I change the item type to text item then it start working.
My question is I do not want to show that item to users so what is the alternative and why dynamic action SET VALUE not working on hidden item.
Another requirement is I want to save the record number of Interactive grid means 1,2,3 with item code so which builtin is used for this.
I think your original issue is fixed in 5.1.1 or 5.1.2 patch release. Not sure which and not 100% sure given brief description. You should try latest APEX version since many IG bugs have been fixed.
The issue of not being able to set the value of a hidden item has been discussed before on this forum. It is a limitation for now and may be fixed in 5.2.
I don't understand the last question. If it is not related to original question perhaps start a new thread.