Your two best options are either Nudge

or  S.U.P.E.R.M.A.N.

and both have pretty good feature sets depending on what you want to do

I second both Nudge and S.U.P.E.R.M.A.N. being fantastic tools.  I favor Super for the simple reason is that you can use it to install the updates with little to no user interaction, which was a factor for one of the clients we implemented it for.

Apple has essentially killed forcing updates. You have to use Nudge, Super, or Erase & Install. The MDM commands are either unreliable or don't provide the users with enough information before the device restarts. Welcome to the wonderful world of managing macOS. 

@bwoods second that. They are awful. Not sure what Apple is doing but getting Mac's update is an absolute pain at the moment.

1.  Configure erase - install to trigger the update from self-service using this workflow.    Once that is done email users and let them know they can and should start updating...

2. Next you can use a simple pop-up box that tells the user they must update which has an action button that when pressed will - trigger the self-service policy above. Link to this workflow here.  Once that is in production for two weeks or so it should work for ~50-70% of your users. *make sure to inventory update often for these non-updated users so they are excluded from the smart group of computers that have not been updated. 

This is a fairly simple way to force users to update. 

3. For the last ~ 30% configure nudge to prompt the user to update. Make sure, it also triggers the policy step one. This is the link to the workflow.   ( this demo config is ugly however, it is functional)

Hope that helps!!