how to solve this problem
Hello,
This error is most commonly thrown when one of the users set to receive the email does not have an email address configured. Could you check the users (TOC > Administration > Users) to make sure they all have an email address?
If you run into older forum posts like this (more than a year old) in the future that don't answer your question, I'd recommend making an entirely forum post instead of responding with a comment to the older post. It will increase the visibility of your question to everyone in the community.
Chris