Automation to send an email produces "'to' field is empty"

Hello! I’m trying to make an automation where an email is sent to a linked record whenever a specific field in a record changes. My trigger tests fine, but I can’t set up an action because whenever I try to test it tells me the “to field is empty.” I have a lookup field to pull the emails for the linked records, and because I read that lookup fields don’t work with automations, I created a formula field that’s pulling in the emails and that’s what I’m trying to reference with my action, but for whatever reason it won’t recognize the email in the field.