What playback device are you using and i am assuming you are not using a Mobile Sports Membership on a smartphone.
Currently watching the South Africa v Australia Rugby Championship match on Sky Sports Mix with both my Roku player and NOW box with my monthly NOW Sports Membership.
New Zealand v Argentina was on Sky Sports Arena at 8am this morning.
South Africa v Australia is currently live on Sky Sports Mix.
If you missed both games, NOW should add them to their NOW on Demand service within a period of between 24 to 48 hours going on previous Rugby Championship games.
The New Zealand game has finished and the South Africa v Australia game has about 10 minutes watching live on Sky Sports Mix.
Here's a screenshot taken from my PC internet browser for the live game on Sky Sports Mix.
Because you missed the game you will either have to watch it on a repeat broadcast (if it is repeated where you will have to check the Sky Sports TV Guide online) or wait 24 to 48 hours when it is eventually added to NOW Sports on Demand under the Rugby Union on Demand Sports section.