Hi
In PRD we assign accounts for price difference. It is independent of ML activation and has no relevance to ML.
PRD can be used with/without account modifier. If you use account modifier, you can assign separate account for price differences from purchase order or production order etc.
In particular PRD- PRA is for price difference in case of goods issue.
In your case, since you are cancelling Production confirmation, goods issue is happening for the finished good (presumed it is maintained with price control S and backflush is 'ON' for this)
This is the reason error is showing up.
In normal cases PRA will not picked because goods issue is generally done for raw materials which are maintained with price control V.
Generally PRA can be assigned with the same account as that for normal PPV (purchase price variance) account unless business specifically wants to assign something else.
Hope this helps
Regards
Abhinav