Accessing the next Unit without completing the previous module

I have created a 10 Unit I haven't published the course as yet. I'm in the process of testing all the Units and I have noticed that the Dashboard is showing some of the Units can be accessed without completing the previous Unit.

I want to set it so the previous Unit must be completed before the next Unit can be accessed. Where is the setting for this? I have created each of the Units the same way and I'm confused why some of the Units can be accessed without completing the previous Unit and other Units can be accessed straight away.

I look forward to your response.

Thanks