You want to create a terrain but you have only one source of input: either an elevation model or an orthophoto. It this case it is still possible to get your data in Virtual Surveyor.
To create a terrain without imagery or elevation, just follow the standard workflow for creating a terrain but leave the 'Input image information' or 'Input elevation information' section blank.
The illustration below shows the settings for an elevation input only.
NOTE: It is important to know that the 'Create terrain' button will become enabled as soon as the minimum input in complete.
Only an elevation model
The Slope Colors will be enabled automatically after the terrain creation process completes with only elevation information.
You can also switch to the Slope Colors display through the middle quick button on the right side of the Drawing group.
You can also set the parameters for the Terrain Map in the Analysis tab.
Only an orthophoto
With only an orthophoto available, a terrain will be created that is flat as shown in the illustration below. By default, the elevation will be 0 meters (or feet depending on the coordinate system of the input file).
Although your project will be flat, you can still draw points, lines, ... but all those geometries will have 0 as its elevation.