Please post screenshots with how your Mailchimp Zap action step is configured in order for us to have more context.
Sounds like you either have the wrong dynamic variable mapped OR the mapped dynamic variable has a blank/empty/null value, thus causing the error because the required field expects a value.
I have a text field labeled “first name” in gumroad during customer checkout. I then use this data for mailchimp and it gives me an error which is also attached.
When you see the {{placeholder_variable}} in the DATA OUT for a Zap Run step, that means either the value was blank/empty/null or the field was not mapped correctly.
Some apps send first/last name, some apps send full name.
If an app sends full name and you need to split it into first/last name, then you’ll need to add another step.