From the Layout tab do the following:

The newly created site can be dragged to any position on the layout, it will usually be necessary to name the site if you don't wish to use the Park IT default site number.

When naming the site be careful not to duplicate any existing site name e.g
In this panel, Park IT site 397 has been named 'F44' - only Park IT site 397 can be named F44. If you name another site in a different category F44 then bookings will jump to that other site.