Why is my 2018 Chevy silverado volume not working?

It sounds like You're running afoul of the automatic volume setting in Your 2018 Chevrolet Silverado. This is a radio setting on the menu, and it's function is to try to adjust the volume automatically based on the vehicle's current noise level or speed. If this is engaged, Your volume's working, You're just unable to control it, and it may be set to "Low". Additionally, there's a separate function to control whether the volume can be changed by teen drivers. Have You disabled these If so, then verify whether You can change the volume using the radio's separate remote; if so, it's an issue with the physical button itself. If not, it's likely that there's a firmware issue with the radio's control software, and You'll need to take it in to Your local shop.