What am I doing wrong?

Being a support is to being learn to die correctly

looks to me like you didnt do a good job supporting your velkoz and/or rumble and didnt focus objectives to win the game quickly when you got ahead.

judging from your teams large number of deaths and your 0 deaths you also seem to abandon teamfights early and leave your team to die, which is pretty bad behavior for a tank support.

If your team has 41 deaths and you have 0 as a melee tank, I'd say the issue probably stems around a lack of aggression from you tbh. As someone that plays Amumu I've eaten deaths that got a 5-1 teamfight win going in KNOWING I was going to die on a very regular basis to the point I'd say most games I play with him has that, or at least majority skirmish victories where I go in knowing I'm dead.

Preseason 9 with Amumu I came out with 70% win rate over 28 games (and we know how weak he is) but you better bet I died a hell of a lot while doing it. I even averaged more deaths than kills. 4.7/5.0/12.3

https://i.imgur.com/dIpIxrf.png

Try Mobalytics, it'll tear your stats apart, but again, I highly suspect it's going to show your fighting stat at near 0 https://app.mobalytics.gg/r/rf/DLZ2Xe

In any case, outlying games shouldn't be the ones where you ask "What am I doing wrong" close games are the ones where you should ask it. Everyone knows League has a lot of games that swing heavily in one direction or another, those are the absolute worst games for analyzing your weaknesses.

You dindt push while you were ahead your kayn build was bad and you needed to focus veigar and then the squses