Welcome to the Make community!
If you are referring to this topic, ✉ Automate Follow-Up of Email Leads a Week After,
Manually type it out like this: {{1.to.address}}
This is because the email module sometimes return it as a single collection, or an array of collections (if there are multiple recipients).
It’s probably a bug but I can’t bother to report it
Hope this helps! Let me know if there are any further questions or issues.
— @samliew