The first time building develop in Qt will take a long time on ubuntu.
Turn off your computer’s suspend after 30 minutes, etc. in power settings.
But good news - all subsequent builds will take less time.
So build the first time through: from top menu Build select Run Qmake then Build All.
And wait. Watch a movie.
For subsequent builds, clean out previous results first: Build/Clean All, Run Qmake, Build All.
Note: If you run ‘Clean All’ before your first build then you’ll get an error message, don’t worry just ‘Run Qmake’
Then when it finishes, Set up the lower left column “monitor” icon: 
Select Release, and “valentina”.
Then run and test LibreFashion by clicking on the Green Triangle.