Hello
As stated on note 1979510, this error message is usually triggered if you try to convert an order for which the conversion indicator is missing.
Therefore, I would not expect to see this error message being triggered during the MRP planning on transaction MD50.
Is it possible that you have any custom code where the planned order conversion is triggered during the MRP run? If you are using strategy 50, then VP planned orders are generated and the order cannot be converted, so the error is justified.
You may follow the procedure from the following document to find out where the error is triggered and check if there is any Z-code on the call stack:
How to analyze an error message in debug
BR
Caetano