The employee is unable to log in to Simployer Classic

You likely haven't granted the person application access to Simployer. 

Search for their profile, then click on "Settings → Roles" in the top menu. Check if the employee has access to Simployer. If they do not, you can grant them access by click on the "+" sign on the page and add Simployer. Here, you can also add other applications if available, such as handbooks, deviations, etc.

You may also check that the user is logged into the right user. This is relevant if the user recently started working in your company and have had a Simployer user before.