It seems you have worked it out, almost.
There is an issue with using the SLSTR data from the command line.
I’ve noted it here:
https://senbox.atlassian.net/browse/SIIITBX-238
You should explicitly define which reader you want, as there three available. Ashish mentioned this already.
That Rad2Refl is complaining about the input product, might be caused by a SLSTR product in 1KM resolution.