It has to do with the type of environment you choose before you start your park, they all have different sets of landscape textures available, limited to 8 textures per park environent. If you already placed some items and if you don't want to start over, go to your menu and select your park in scenario editor mode. IMPORTANT: There are two buttons for the editor mode, you will need to select the small one with the paintbrush, this will take you to a selection menu where you can freely select the landscape textures you want/need. My explanation might be a little complicated, however, if you have problems i can post a run-down later.