Prerequisite is already running socat - UNIX-CONNECT:/var/run/tor/control.
It’s not installed. It’s uwt.
dpkg -S /usr/bin/onionshare
Please use Potential Solutions Beyond Whonix! to learn how to install onionshare or any flatpak based application inside Qubes. In this regard, Whonix is same as Qubes Debian template.