It looks like the acrordrdc snap does not plug the cups-control interface, so the snap author would need to add this plug to their snapcraft.yaml for you to connect that interface.
Unfortunately the snap author did not add contact information to the snap description, however the LP author seems to match this repo on GitHub, so you could try filing an issue on that GitHub repository asking for help adding this to the snap (and also maybe for them to set a contact setting for the snap, even just to the GitHub issues page is enough I think).
Also, to clarify, the
--classic flag only allows installing a snap that has been published with classic confinement, it does not change the confinement of a snap. The acrordrdc snap is not published under classic confinement, so this flag does nothing:
$ snap install acrordrdc --classic
Warning: flag --classic ignored for strictly confined snap acrordrdc
acrordrdc 2018.011.20063 from Taqi Raza (mmtrt) installed
If you saw somewhere online in a tutorial or something that this flag changes the confinement, could you let us know so we can try to get that information corrected? Unfortunately this is a common misconception we see with some frequency.
Hope this helps!