For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:aggplanta/openSUSE_Tumbleweed/home:aggplanta.repo
zypper refresh
zypper install qtractor
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:aggplanta/15.5/home:aggplanta.repo
zypper refresh
zypper install qtractor
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:aggplanta/15.4/home:aggplanta.repo
zypper refresh
zypper install qtractor