This setting is not directly related to the scheduler itself, but it is associated with how your employees should perform tasks. Therefore, we decided it's appropriate to mention it here when you are configuring and planning tasks.
Sometimes it's crucial for a business process that employees cannot start the next task without completing the previous one. Occasionally, employees might try to be clever. To avoid a task being marked as overdue, they come, start it, and then go to some other place and start another task there. Yes, the system by default allows starting several tasks in parallel. It's a normal scenario when an employee comes to clean a meeting room, starts the task, and then people come in, asking to finish later. What should the employee do? In such cases, it's very useful that the system allows starting other tasks while some are still open. So, it's up to you to decide. The most important thing is to carefully think through all the pros and cons.
If you have firmly decided to enable this setting, go to the "Settings" section in the side menu and turn on the toggle "Prevent employees from starting a task if there is work in progress".