Nomenclature - Transmit
In Preference>Playback it's listed as "enable mercury transmit"
In Keyboard Shortcuts it's listed as "enable transmit" (so if you search for "enalbe mercury transmit" you will find nothing, because the search is very literal)
But actually, the command you can use in Keyboard Shortcuts is a toggle, not an enable.
Pick a nomenclature and stick with it.
(There's probably other examples of this.)