Hello @Seller_aVGmvnIXPnOhw,

To resolve error 8040, you must get approval to create new variation relationships between ASINs for a specific brand.

To request approval, contact Selling Partner Support via the Get help page. On this page, select Adding a product as the issue type, click Received an error code or message, and then select Error 8040 – Match only variation error. Then, fill out the form and submit it.

More information on Error 8040 help page.

Adam