While working my way through the trailhead module Work with GeoJSON and Create the Map, I was stuck on the following instruction:
To create a custom map type, click (paint-brush), expand the Map section, and then "+"  next to the Map Type property.
Unfortunately I don't have that "+" sign.
Can anybody help me in how I need to create that custom map?

While working my way through the trailhead module Work with GeoJSON and Create the Map, I was stuck on the following instruction:
To create a custom map type, click (paint-brush), expand the Map section, and then "+"  next to the Map Type property.
Unfortunately I don't have that "+" sign.
Can anybody help me in how I need to create that custom map?