I have searched, but have not found an answer to this on the forums:
How are RETAIL materials extended to new plants? These aren't standard materials, so the MM01/MM02 transaction does not apply. The retail transactions for material create/edit are MM41/MM42.
In MM42 I have used the validity to choose the plant in the purchasing tab:

However still getting the error in ME21N:

Any ideas? Thanks for any help!