If you see the screen below, there is already another session with the same user:
The Z Workforce doesn't allow the same user log more than once on the panel simultaneously. The system is configured this way for security reason, avoiding that a possible error can not be audited.
When the user tries to log in to the panel and another session is already open, an email will automatically be sent to the user with a link that, when clicked, will end the pending session.
Therefore, it is imperative that every panel user uses a valid email when creating their account.
If you have not received your email, check your spam box.