Inside Leaflet module if you select Stadia as tile provider you get 401 error
on layout
All you have to do is add apikey="<your api key"> attribute to your leaflet map component 
On development no authentication is required, when you publish it online you can add the domain in the client dashboard from stadiamaps to allow access.
Raster Map Tiles - Stadia Maps Documentation
Domain-based authentication is the easiest form of authentication for production web apps. No additional application code is required, and you don't need to worry about anyone scraping your API keys. We recommend this for most browser-based applications.
- Sign in to the client dashboard.
- Click "Manage Properties."
- Under "Authentication Configuration," click the button to add your domain.
We currently don't have an API key option on the leaflet map component, we can add it when it is needed.