Do you create an instance of TApplication in your main? If not, auto-loading of ROOT libraries will not work and you will need to load (some of) them manually, i.e. ‘gSystem->Load(“libTree.so”);’.
Do you have the “/etc/root” subdirectory? If yes, you most probably have Ubuntu’s default version of ROOT installed, which may interfere with your “private” ROOT installation (in “/opt/root”) -> “completely remove” all standard Ubuntu’s ROOT packages (using a “package manager”) - see also [url]ROOT 5.32/00 support