Dstv now website unable to mirror from MacBook to Apple TV ?

The DStv Support FAQ only mentions allowing AirPlay from an iOS device to Apple TV: “Can I use Airplay to stream DStv Now content from an iOS device to a TV? Airplay is available when using an iOS device with the latest version of the DStv Now iOS app. For Airplay to work, the source and target devices both need to be on the same Wi-Fi network. The Airplay icon will become visible once the video has started playing.”

They may simply not allow it from the web browser, which might be a way of securing content, and related to licensing. Apple does not restrict this, the content provider DStv does.

Where does that error code appear? Some content could be restricted for mirroring, but codes may be site specific. For errors specific to DStv service, you probably should be consult them or their forums.