Simple question. How do I cut sound in After Effects 2021?

With a layer selected you can hold down the Alt/Option key to reveal the Audio Levels property and set a keyframe. Move back or forward 1 frame and set the value to -192 to turn off the sound. If you press "l" then Shift + "L" you will also see the audio waveform and this will give you a visual indication of the audio levels.

You can also see the Audio Levels in the Audio Panel which is available in the Default workspace.

You can also split a layer at the CTI (current time indicator) by holding down Shift + Ctrl/Cmnd + d and turn off the audio (Speaker) icon in the layer to mute audio. 

You can also use Alt/Option  + [ to set an in point for a selected layer or Alt/Option + ] to set an out point for a selected layer. :"Selected Layer" is the critical part of any of these operations. 

Another really good idea would be for you to spend a few minutes in the Learn workspace figuring out how the basic UI works and then spend some time with the User Guide figuring out some basic workflows and rendering techniques. Watch out for tutorials you find on YouTube. Most of the new ones are poorly explained recipes that an enthusiast has stumbled across that don't necessarily use efficient workflows. Vet your trainers. Do some homework and you will catch on pretty quickly. Poling around in the UI and asking questions on forums is usually the long way to the park and sometimes the path can be pretty scary. 

Those shortcuts only work when you are using an english Keyboard. You can customize these commands in Edit > Keyboard Shortcuts. Another way to do it is to select the layer, place de CTI (Playhead) over the frame you want to trim and use Edit > Split Layer. It divides the layer and you can delete the portion you are not going to use.