In the Generate Longitudinal Feature command, I am unable to select the graphic that I wish to use as my primary element. I click on the line, but it will not highlight.
The most probable cause for this problem is that the Locate Lock is set to "features". Toggling the Locate Lock to "graphics" will likely resolve the issue. The Locate Lock can be found in Tools > Locks.
The lock applies when you use a locate button to specify a position in the graphics file. (Locate buttons are found throughout the interface. Each button is indicated by crosshairs icon.) The Locate lock determines whether to snap to graphics displayed in the graphics file or to snap to the position occupied by a feature in the active surface.
If the lock is set to graphics (the icon shows a single, red line), locate actions will seek the nearest displayed graphics.
If the lock is set to features (the icon shows an image of a surface), locate actions will seek the position of the nearest feature in the active surface whether or not the feature is actually displayed. You can use the locate lock to locate features that are not even displayed. If they exist in the DTM, the locate will find them.