Approval Workflow - Email Cannot Be Sent Error

I am receiving the error message: "The email message cannot be sent. Make sure the outgoing email settings for the server are configured correctly"

This is happening on an out-of-the-box Approval workflow in a regular team site on my SharePoint Online site.

The workflow is trying to send email to 2 users, both of whom are valid users in Office 365 with the email attribute set on their profile to the Office 365 email account. Both users can access their general online inboxes through Outlook online. Both users are global admins.