Unable to convert the Manufacturer barcode

Hi,
I am new seller on Amazon and currently doing wholesale FBA in a UK market. I have enrolled a product under small and light programme but unable to change the barcode preference. I have changed the default setting to Amazon barcode. However, when I create a shipment it does not give me the option to change the Manufacturer bard code to Amazon barcode or FNSKU.
I am getting the following message.
On the first page, it now says Manufacturer Barcode Only.
Clicking on Why: this product is not eligible for tracking with an Amazon barcode for one of the following reasons:
• This SKU is already assigned a barcode preference.

It looks like to me I can’t use Amazon barcode for this specific product. I tried multiple times to change from FBA to FBM so that I can get the option of Amazon barcode but nothing changed and I always get the same option of Manufacturer bar code.
Thanks