Check your linked data fields for the "Brand*" field of the catalogue. Brands can most easily be linked via already existing manufacturers (Tab: Global) of the article, or via properties (Tab: Properties) of the variants. Therefore, check that there is a link to the variant in each case. If you want to link via the manufacturer, select Add data field > Manufacturer > and your desired manufacturer.

It should also be noted that the linked ID of the OTTO brand should be selected in the dropdown. Manual entry of the ID should not be done.
An example mapping:

If mapping is based on the manufacturer name, the Comparative Value must be specified:

This value corresponds 1:1 to the value stored on the variant.

Custom values should only be used if no other brand is to be transmitted. Otherwise, all other underlying mappings will be ignored and only the custom value will be transmitted to Otto.

If your brand/ID does not exist in the OTTO brand field (left), you can have the brand registered with OTTO Market. More details can be found in the Helpdesk at OTTO Market.