Duelingbook Forum

Normally it should show you how many cards in your opponent's Main Deck and Extra Deck but in this duel it did not show me.

This is a problem because for example if I play a card that requires me to know how many cards in my opponent's Main Deck or Extra Deck, like That Grass Looks Greener or Re-Cover, I won't be able to get it to work.

https://imgur.com/hhZUAqb

That seems unusual. It should be public knowledge.
Because it is, your opponent should answer honestly when asked about it. If in Rated, you could ask a judge to confirm it.