Create Accounting asking to setup Offset Account for WIP material Issue

We are using perpetual average costing. The create accounting is given error as : The input source values don't map to any output value defined in mapping set Offset. Details: You must either define a default output for mapping set or specify mappings for all possible input values.

Similarly we are getting this error for In-Transit Interorganization Transfer- Shipment to In-Transit transactions.

Error: The input source values don't map to any output value defined in mapping set Offset. Details: You must either define a default output for mapping set or specify mappings for all possible input values.

I want to understand why the offset account is required. In which cases it will hit the offset account. How to avoid this issue.