well, which version of LibreCAD do you use?
The distance measuring action simply produces output of measured value into Cmd widget - please check whether some output is present there.
In 2.2.2-alpha (
https://github.com/LibreCAD/LibreCAD/releases/tag/2.2.2_alpha-latest) there is a "Informational Cursor" tool that provides indication of distance between points in visual way near to cursor.
Something like:
As for the second question - sorry, I not sure I got your point. Which command do you use for creation of line?
Btw, are you sure that your current layer is not hidden? Potentially, if you don't see the line - it might be created, yet if the layer is hidden - it will not be visible.