Don't reinstall Xcode 11 for now.
Vals-Mac-mini:~ valraemaekers$ xcodebuild -version
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance
To fix the error :
Vals-Mac-mini:~ valraemaekers$ sudo xcode-select -s /Applications/Xcode.app/Contents/Developer
Thanks for your help Erel , Buildserver is now running