v9.0.2
When I draw track I’d like the PCB Editor to automatically draw a particular track width according to its net class. In ‘Set Track and Via Properties’ the option to set the track width is locked to ‘-- leave unchanged --’. What needs to be done to allow a width to be assigned to that net class?
Instead of “Set to specific values” in the “Action” in the above image, click on the “Set to net class / …” and “Apply”.
Also in the PCB editor set the “Track” (Top LH of screen) to “use netclass width”
It’s right in front of your nose:
And that are these buttons in the GUI. For me it even sais Use netclass Width 
May I also suggest that every now and then you read a small part of the manual, and experiment a bit with new buttons and functions you can find in the menu’s. It’s a bit of a playful way of learning. Some people like a more structured apprach (real books, video’s, official courses) So pick what suits you.