Likely what happened is that the user was added as a "User" with Worker permissions when they were onboarded onto the platform. This does not automatically create an asset in the system.
An administrator can simply go and create an asset for this person under the "Asset" tab, making sure to use the exact same email address.
Best practice is to always create the Asset when adding a new user. This will automatically create the associated User with Worker permissions.
If you receive this message when logging in, please contact your system administrator to resolve the issue.