How do I enable the 'Export Selection' command?

I’m running version 2.0.5 on Ubuntu Studio, and I’m new to Ardour and to Linux.

How do I export a selection of regions? The command from the file > export menu is always disabled no matter what I seem to do…

use a range tool!

press ‘r’ to activate the range tool and than click at the point you want the range to begin, don’t release the mouse but move it in the time line. you will see the range gets created that way.
than go to export - the option is available now.