system
Given the following post, respond as if you're commenting on Reddit:
human
[40k]Promotions in the Guard?

So I had a pretty cushy job in my planets PDF for the longest time, not much happens here, till it came time for the tithe to be collected and I was lucky enough to fall within the top 10% of the PDF so was recruited into the newly mustered regiments. Fast-forward five years and I've been working my butt off trying to get promoted high enough so I don't have to risk my butt on the front-lines. But after over four campaigns, volunteering for high-risk operations dozens of times but I have only managed to make it to the rank of Sargent. Is it a knowledge thing? would I have to learn about tactics and that kind guff to hope to be put higher up, or is just a matter of time because that's what I have been telling myself but it just doesn't seem to be happening, Hopefully this will reach someone who can give me the proper advice.
gpt
Moving from Sergeant up to a platoon command staff can take a decade or more. More likely, the platoon will be sufficiently depleted to be reformed as a Veteran combat squad and attached to another formation before any such promotion happens. 

Becomming a Lieutenant (IE commander of an entire Platoon) is something that typically only happens at original muster, to fit the new Platoon into the regimental command structure of the Astra Militarum. It is very rare for a Platoon to be restructured and a new Lt. promoted, as typically if the command squad falllls, there's not much left of the unit. 

Exemplary conduct and skill can be noticed by the assigned Commisars, and the individual can be decorated for their actions by regimental command. This can lead to the relatively safe posting of serving in the regimental command unit, from which upward mobility becomes easier.

Basically... Your best bet is to do a superb job in from of Commisars, but you are still looking at a decade or more in the shit.