Unable to use "Remove Silence"

Not able to use 'remove silence'. The box is greyed out. Any ideas what could cause this??
https://forum.blackmagicdesign.com/
https://forum.blackmagicdesign.com/viewtopic.php?f=21&t=220728
Reynaud Venter wrote:Have Gates been defined for the Range?
HaHundo wrote:Not able to use 'remove silence'. The box is greyed out. Any ideas what could cause this??