If you do not see the button to end the class it may be for one of the following reasons:

The class has not started

One of the requirements for the button to appear is that the class has started. The class does not start until the teacher and a student are connected.

The minimum number of minutes to close the class has not passed.

Another requirement for the End Class button to appear is that a minimum number of minutes (usually 20) have passed, here more information.

You do not have permission to close the class

If you are not the tutor of the class, you may not have permission to finish it, here it is explained in more detail.