Hi @chlqhqodlekt
For us to have more info about the Zap step error, post screenshots showing how your API request is configured in EDIT mode with the field mappings visible, and post the link to the API endpoint documentation you used to configure the API request.
Hi there, @chlqhqodlekt 
Hope you don’t mind me jumping in here to lend a hand. Based on the error it seems that the API you’re making a request to doesn’t support a comma separated list of email addresses in the To field. It might be that the API you’re working with needs them formatted as an array or separated by semicolons instead.
Can you send us a link to the documentation for the API you’re making the request to? That will help to give us a bit more context on what its requirements are.
And can you share a screenshot showing what’s been selected for each field in the Configure tab of the Webhooks action:

Make sure to blur/remove any personal info (names, emails, API Keys etc.) before sharing—you can use a tool like Zappy for that.
Looking forward to hearing from you! 