Where you see this Amazon error, if there are no further details such as missing required data (example are Brand, Barcode/UPC, Title etc), then the most likely problem is the Brand value given is not eligible for a new ASIN to be created against it.

Unfortunately ChannelUnity doesn’t have a solution for this issue and so our best advice is to follow the advice Amazon has listed in SellerCentral

Advice from Amazon is as follows:

You may not create ASINs for this brand. Please review our ASIN Creation Policy here: https://sellercentral.amazon.com/gp/help/201844590 . If your product is not listed in the Amazon catalog and should be listed as a new ASIN, contact Seller Support at https://sellercentral.amazon.com/hz/contact-us and mention error code 5461.

Once this has per corrected as per Amazons advice, you should then be able to retry uploading your product via ChannelUnity.