I'm experiencing this problem as well. When I click the button like shown in the screenshot simply nothing happens. Also the 'T' shortcut doesn't show any reaction.
I'm using Studio One Artist (Build 5.1.2.62686 from Jan 13 2021) on MacOS Big Sur (11.1)
Update: Copy/Pasting existing tracks in works as a dirty workaround.
Specs: 16G of memory, 2,8 GHz i7