MSFS Navigation help

Hi everyone. First of all, I am not a Pilot (too old for that) :(

I have been having issues while navigating using the GPS on the TBM 930. Regardless of how I entered the trip information, either manually or by loading a flight plan created @ the SimBrief website, the plane will not adjust the descent altitude while on auto pilot. I was watching somebody on a YouTube video saying that activating the VnV button should make the plane glide to next waypoint based on your flight plan, but on his video the button was not doing anything when pressed. He claimed that it is a bug. I checked on my FP and it shows each waypoint's altitude like it should.

I am flying at the hard mode settings, except for ice and fuel (that will come later ) 8)

My question is, am I doing something wrong, or the VNV feature is not working?

Thank you in advance for your help.