There is no native popup layout in Atlas yet. It is very easy to create one yourself. One placeholder should be sufficient.
Anywhere in your module, not in Atlas UI Resources, create a new layout. Right click a folder in your module, Other and choose Layout.
I just created another, the placeholder is put in automatically. If you're developing for smartphones, you might want to change the canvas width of the popup to 500 for example so it more closely resembles a smartphone layout.
Edit: Use my widget which I created just for this purpose: https://appstore.home.mendix.com/link/app/114262/ITvisors/Native-modal-container
Have you tried the widget “BottomSheet” of the NativeMobileResources (which in 8.8 probably still is called NativeMobileActions). That does not give you a popup, but a page that is sliding in from the bottom of the screen. Looks very nice.
But if you still prefer a popup, you can also upgrade. In 8.14 and up you do have a popup:
When I choose Native as platform and Pop-Up as Layout type you can’t choose any Layout.