Default scheduling mode Project app

On top of what Paul shared below you can also insert the selection box for this setting in your Planner Power App and pick it when you start your project/plan. See image below.

Keep in mind you can only set this once, upon plan initiation. You can't change the mode later, so make sure you have a good idea as to the type of planning and tracking you'll be doing in your schedule prior to picking fixed work or duration. 

You probably don't want to pick 'fixed units' because there's really nowhere in the tool to adjust this for each resource like you can in MS Project Pro or Online.  Planner Premium uses fixed work and duration as the key scheduling modes.  The drop down in the Power App just provides these two options (fixed duration or work).