I am not sure what's the problem is. You can't read a sideline and calculate it without moving the pieces?
If you are stuyding on a real board it's important. If you don't , then it's not that much.
But it is very easy. Start with very simple endgame positions(King and pawn vs King) or some elementary checkmates(2 bishops vs king is a good one). Try to calculate the sidelines then play them on the board. It didn't took me more than a couple of hours.
I'm sorry I've been playing for less than a year and I don't even know what a sideline is. Also, I think you misunderstood my question. I'm talking about calculating like this: https://youtu.be/2R1mIJ80CJg?t=2948
I can read and write algebraic chess notation but since I'm not too familiar with it I can't calculate or analyze using chess notation. How important is this chess skill?