Hi Siccups.
Thanks for reaching out to Method Community!
It sounds like you're experiencing an issue with creating an opportunity via the Method API/Zapier, specifically regarding a required field "Customer".
There is a help center article that provides guidance on how to create an opportunity using Zapier in Method CRM, which includes information on how to correctly set the customer field. I would recommend reviewing this article and following the steps outlined there to ensure that you are correctly mapping all required fields, including the customer field.
Regards,
Inder