Hi Everyone, I need a help for soln by given below screenshot. Pls suggest, how can I write this formula by using this method. Thank you.

Change the output type? Guessing brand name isn't an integer. You also may need to look at the access set on that custom field.

Availability of fields depends on the starting record type and available relationships. Are you sure vendor is an accessible record? If so, verify the field setup as James suggested.