Unfortunately, although the Mail merge stage allows you to select a hard-coded email address, this option doesn’t currently work.
When you are trying to send a Mail Merge to hard-coded email address, it fails to do so. It does not stall, does not display any errors. It just does not send email.
What you need to do is to create a new user record and assign the required email address to this user account. On the mail merge stage, you can then select the “E-mail User” option and “select from a list of users” and select the user account you have created. The mail merge will then be sent to the required address.