If you have a schedule saved, it won’t let you delete all the blocks. You HAVE to have at least one schedule block on one day. It is mandatory to have a schedule.
You have to delete the schedule itself, not the time schedule blocks. When you open to Schedules, don’t open that schedule to Edit Schedule. Just swipe the schedule left to reveal the red trash can and delete it there. You should then be able to delete all your schedules.
I have transitioned to using just the preference temps and Alexa Voice Commands (Alexa, I’m Home, I’m Leaving, Bed Time) to change the Thermostat State. I posted that workaround here.