The reason why you're getting this INVALID_MSKU_REMAP error when trying to add items to a SellerChamp manifest FBA shipment is that:
The SKU / ASIN within the SellerChamp manifest FBA shipment does not match the SKU / ASIN within Amazon seller central.
You're trying to replenish an item that has been inactive for sometime.
There are a few ways to go about resolving this issue.
Check your Amazon seller central inventory and remove the SKU that is causing the conflict
Update / change the ASIN for the SKU within your Amazon shipment
If your SKU / ASIN match between the SellerChamp manifest FBA shipment and your Amazon seller central:
Simply duplicate the item
Remove and delete the item that is causing an issue, entirely from Amazon and SellerChamp.
Following any one of the above resolution should get you going. However, even after confirming that everything is okay and both of the above resolutions failed, then:
Please contact Amazon support for further assistance.
Wait 12 hours for the error to automatically get resolved.