The way this action works is that whenever the zap triggers, the currently running timer inside of Harvest is stopped. Since you can only have one timer active at a time, Harvest figures out which one to stop on its own.
This action is useful for zaps like GTalk -> Harvest, where you could send a chat with "#clock-out" as the message, which would stop your harvest timer.
Harvest requires a user with "admin" permissions to be able to connect via their API. If your account does not have sufficient permission, you will get this 404 error when you try to connect. You can read more about user permissions on their help docs here.
Right now, there is no way to access all user information in one Zap. You will need to connect each user's account and create a separate Zap for each one at this point in time.
You need to have a timesheet entry created today in order for it to show up in the trigger. To resolve, create a new time entry in Harvest and retest the zap.