Putlistingitem to add an offer to an existing ASIN

[astanley44]

I am trying to add offers to existing ASINS using putlistingitem.
I have LISTING_OFFER_ONLY and productype as PRODUCT
It works in sandbox but I get this error on live

Creating products is not supported with the ‘PRODUCT’ Amazon product type. Either specify a specific Amazon product type or specify an offer-only requirements set.

I assume its either LISTING_OFFER_ONLY not working or it's no longer allowed for the PRODUCT producttype and I need to use the correct producttype for each product and associated required attributes, which is an absolute nightmare.

Can anybody confirm either way?