I've had rather a lot of trouble with this. I run OpenSuse. There are rpm's for Qucs in there repo but they are bugged. Simulation brings up this error message
/usr/bin/qucsator: undefined symbol: redirect_status_to_stdout
So moved on to compiling. This generated config errros Qt > 4.x on some and must be Qt 4 on some other versions. I fixed this by addin Qt5's Qt4 support dev files to my installation. This just meant adding libqt4-devel to a default installation which also pulled in several other files. I used YAST so don't know what those were. Life with KDE is much nicer on OpenSuse stable and always is. I've used it for 20+ odd years. Posted as I have seen comments that qucs can't be run under qt5. Currenl schematics are working and a sample file did simulate correctly. This is with Qucs 0.02. Mixed results with the others.
One oddity that may or may not be a bug. I've lost the very convienient probe symbols used in all of the examples and now have several 2 terminal "meters". I may have had the nice ones as I installed over an opensuse rpm install as no version I have compiled from scratch has provided them. I've attached a jpg showing the symbols I am refering to. 10pf is unwise in audio range stuff and I want to simulate to see likely effect of strays.
I found the wire lables.