Just to clarify, is your S9 Plus rooted or not?
If your phone isn't and this is apparently a false negative matter, try going into the Settings >> Apps menu, find and open the Showmax app listing, tap on Force Stop, and then find and tap on 'Clear data'. Using Clear data removes all that app's settings and config files, along with wiping it cache. This returns the app to its first time used state so now when you start it up you'll need to re-enter your login and password info. Something in the app previously mistook the not-rooted state of your phone.
But if your phone is rooted, a lot of online services (streaming content sites, banking sites, etc.) are averse to working on rooted devices because of the pervasive problems of piracy (DRM can be more readily bypassed) and security (interactions involving financial matters need to be tightly curated).