I just ran into the same problem on a fresh install (5.0.0.72 on XA2 Ultra); apparently the storeman package itself (so not the installer package) requires the system to have had one full repository refresh before it installs.
I’m not sure how SFOS handles it internally, but adding my account and launching the Store once was not enough, I had to run devel-su pkcon refresh. After that Storeman installed through its installer.