How to Create a New Slide:
In
Edit Mode click on the 
Add button in the top left select "Create New Slide". Select any Template based on what your vision for the new slide is going to be.
From the Add drop-down menu, you can also add a Slide from a few other locations as well if you do not want the slide to be blank. You can choose from:
Add Chapter From Master Deck Library
Add Slides from Master Deck Library
Add Slides From Presentation
How to Create a Blank Slide:
In
Edit Mode click on the Add button in the top left select "Create New Slide".
Search for the "Blank Slide" Template which will create a fully blank slide once loaded.