My first post here, not sure if the formatting is proper and if I need to provide more information. Please let me know if any correction is required.

In this position 18/16(2) is obvious and I played 6/2 with the rest. However XG says it is a medium size error and the correct play is 4/2(2). Could anybody please explain the reason for that?
Thanks in advance.

The reason should be that he keeps one more attacker in front of your anchor. The position for black is after 6/4(2) much robuster than after 6/2. White , in contrary, is very stripped and likely to give up something soon.

A general rule of thumb in all wars is to keep your ammo as long as you can

I'd explain it this way. After 6/2, your checker on the 4-point is 'semi-dead'. Unless you actually roll a 2 and play 4/2, that checker won't play any further role in the game.

After 4/2(2), all your remaining checkers are fully alive and working on the 4-point, which represents a slight improvement.

Don't feel bad about missing this play. Prior to the bot era, everybody in the world would have made your play, because "the 4-point is more valuable than the 2-point...", and thought nothing about it. The bots taught us to think more clearly about what it means to keep checkers fully active.

Donby, Wamy, Bill, thank you for the explanations! Great forum, by the way.