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

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() 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.