Fly on unoccupied land (it is marked "free land" on the bar above).
Right-click on the land and choose the menu LAND > INFO
Click on "Rent 1 Day"
Choose domain NEW and click PAY (don't panic, you don't need money, everything is free on Planet)
The ground is then automatically erased because at the beginning your land is completely empty.
To continue, read the chapter : place a ground