When submitting non FBA orders to FBA, you need to take into account that FBA has to recognize the order data to be able to complete it, so vital part is making sure the SKU you are submitting will be recognized by FBA.
If the SKU is not identical to your FBA SKU, then you need to use Fulfilment SKU as an extended property so that FBA can easily recognize the right item.
The following article has more information about it:
Does anyone successfully use Linnworks to place outbound orders for Small and Light FBA items?
We've tried and the system errors, we've logged the error and followed the suggestions from the support staff however this resulted in a different error with no fulfilment achieved.
Errors we receive:
ERROR: the SellerSKU for Item Id:[ Small & Light SKU] is invalid.
Error after removing Small & Light SKU as directed by Support Staff
ERROR: Value SupplyCategoryUnavailable for parameter MerchantSku is invalid.
The support staff are on their normal 24 response cycle so just trying to speed up the resolution process or even confirm other people can use Small and Light FBA for outbound orders successfully.