Any reason why the send email node won't work in this situation?

You can try toemailaddress.

Try : toemailaddress()
For reference documentation link is forum.appian.com/.../Conversion_Functions.html

toemailrecipient() doesn't take text as input. You can pass User or Group as input. If you need to pass text, use toemailaddress() as Akshaya and Rishi suggested.
Also, in general, when you don't have enough information about the error from Alert message, you can look at log files such as application-server.log to find more detail about what caused the error.

toemailrecipient only accepts 3 Appian types as input: email address, user and group.

forum.appian.com/.../Conversion_Functions.html

To use toemailrecipient on a text, you have to first wrap it with toemailaddress(). See the attachment.

Or as others mentioned just use toemailaddress() and no need to wrap it with anything, it will just work too.