I cant edit under the video track

pls help me

It’s impossible to determine what you are asking. That video explains exactly nothing.
Please try harder to explain the issue you are having and also include details of your Shotcut version, operating system, PC specs and the source file type.

sometimes I can not edit with “the white line” under the videos. From time to time it goes normally, but now it doesn’t work and that is frustrating. I have Shotcut version 18.03.06 (the newest) and my computer working with Linux Mint

Sorry, I really have no idea what you are referring to.

Last is 18.06.02
And what do you mean by “Edit” ? Cut? Add filters? Move?
Do you double click the part you want to change first? Do you “switch” between Source and Project?

I see the problem you have in the video, but I do not experience it. It looks like a Qt OpenGL problem - that the OpenGL Qt generates for the Timeline does not play nice with the OpenGL implementation (libraries+driver+hardware) on your system.