Posted by dellus on Nov 17, 2019; 9:23pm
URL: https://forum.librecad.org/Tools-Not-working-tp5717969p5717976.html

While trying to reproduce your problem, I have encountered linetype "No Pen". You can draw with it, the drawn entities exist, as they are shown when selected (Select all), but are otherwise not visible, not even as placement preview. If your linetype is "By Layer", still in layer properties "No Pen"could be set. Unlikely this all could happen accidentally, but who knows.
Could also be other layer issues. When you do a copy (what works for you), it normally stays on the origin layer, and also keeps the pen properties. Parallel line produces a new entity with present properties. On a different layer? Locked? Set invisible?
As you are experienced with CAD one would expect you will see these things. But sometimes one misses obvious things. Can you arrange someone else with CAD experience to have a look on it? Sometimes an other person recognizes something at first glance where one self is blind to.
You could provide here a sample drawing file with that 100x100 square, and a screenshot of your whole screen, maybe also of the application color preferences. Perhaps someone will see the reason.