Please see below common OnBuy error messages and how to resolve.
Product code '7107727881356' is in a reserved GTIN group : invalidRequest, Product code '7107727881356' is in a reserved GTIN group- EAN is not valid.
This error message is generated when OnBuy identifies these barcodes as reserved range barcodes.
These have been reserved by GS1, the global authority for barcodes, meaning they have not been released for use.
Can you check that you are using the correct barcode for this product.