Learning how to navigate the 3D space when working in Zbrush can be confusing at first, but it’s very easy and intuitive.
» First of all leave Edit mode by pressing the T key or pressing Edit on the main interface.
You need to be out of Edit mode to be able to clear the layer.

» Clear the canvas (layer) by pressing the keyboard shortcut Ctrl+N.

>> Go to the Tool> docked panel and choose the Polymesh3D tool.
>> Drag/Draw the tool onto the canvas once. Immediately press the T key or Edit button to enter Edit mode.

You need to be working in Edit mode to have the ability to rotate, zoom and pan your 3D mesh.