I am selecting audio which I want to “silence” however even though selected, (having highlighted by dragging over the selected section), when I go to Edit; Remove Audio or Labels; Silence Audio, the Silence Audio (and all other options) are greyed out.
Most operations in Audacity require that Audacity is “stopped” (not playing, paused or recording).
I’m betting that you “Paused” playback, so please try clicking the Stop button.