Why would an ALC class be cancelled for a soldier (I just received an email saying "cancelled in a substitution transaction")?

It could be the class is filled and if it was an MTT it could be due to funding and last your unit could of pulled you out of the class without your knowledge

Your unit pulled you out of the course, and replaced you with another Soldier.
As to why, your CoC would have to answer that.
Not sure what your climate is there, but if I were to venture a guess, you might know the reason already.

You seat was taken and given to someone in your command because it says cancelled on a substitution transaction. I would not buy that point thing because if you were eligible it shouldn’t affect you unless you were there to fill an open slot ( mean holding it). Not a good justification from whoever told you that