Volvo 2020 , code 256000. ?

My 2019 had the same critically low fault. It was low, but hardly to the point I'd call critical. I dumped 2, normal sized 17 oz. Water bottles into the reserve tank and got rid of the warning. Then went to the nearest truck stop, got a gallon of coolant and dumped about half of it in to fill up the reserve tank.

 

Now that you have that code and you are still consuming coolant, your EGR cooler has probably failed. I had to replace mine at 125,000miles after getting that code and continued coolant consumption for a couple of weeks before it could be diagnosed.

 

In my case, when the coolant level was good and it was showing "coolant level critically low", even after replacing the sensor, it turned out to be the plug itself (to which you put the sensor in) that went bad. So I wired in a new plug and the problem was solved.
However, since your truck is 2020, it would seem a little odd for those plugs to start giving out so early, so it still might be something else.

 

True but I would still test it. Lots of new or nearly new failures in parts these days. My wife's new ride had a coolant leak at 3,000 miles.