So you can't create a backup with XOA Free. This edition is meant for all usual administrative tasks, like XCP-ng Center if you like.
If you want backup features, 2 choices:
going for XOA Starter, Enterprise or Premium depending on your backup needs. See each edition features here: https://xen-orchestra.com/#!/featuresmatrix (each edition comes with pro support, updater and remote SSH access if needed)
install XO from the sources on a system you like, following our guide: https://xen-orchestra.com/docs/from_the_sources.html