Scheduling clocks with adverts involves setting up the clock, scheduling the clock and generating the log.
Create a clock with a fixed time marker at the start of the hour. Then create soft fixed time markers at 20 minute intervals:
The gaps you have not scheduled between fixed time markers will be filled with the playout pattern (Manage > Playout Pattern)
Schedule the clock via Manage > Clock Schedule... daily every hour:
Schedule the log by choosing an hour and and clicking Schedule: