En passant not available for some reason

This game lichess.org/HvEF5q5l/white#29
move 15, why there was no possibility for me to use En passant rule, pawn b7 ?

Thanks.

Your pawn needs to be on the FIFTH rank, your pawn was on the SIXTH rank

that's not how en passant works, your pawn has to be on the 5th row, not the 6th.

You can capture en passant only in your half of board?

Your opponents pawn has to be next to your pawn for en passant to work. Your opponents pawn went past your pawn.

So have I understood it correctly that the pawn should be on a rank 5 or lower to use the En Passant rule?

Opponents pawn make the the big two-square move and your pawn immediately captures the middle square (of the move, the one on the 6th rank) - and the opponent's pawn by en-passant, too. To do that your pawn has to be on the 5th rank, yes.

The "en passant" rule is more easy to understand if you know why it was introduced.

At first, pawns always could move only one square. Then, to accelerate the contact between the armies during the opening, they were allowed to move two squares, but only in their first movement.

The "en passant" rule was introduced to prevent this new movement from allowing a pawn to skip the possibility of being captured by a colleague of the opponent.
That is, if the pawn could have been captured by another pawn having moved a single square, instead of two, you can capture it as if it had effectively moved a square.
The only thing that needs to be added is that it is only allowed in the immediately next movement.

#1 Hears a link to further educate you on the "En Passant"rule. Also...have you completed the Lichess basic chess course? If not click on the learn tab and select "Chess Basics" Take the course...It will help your game. :] en.wikipedia.org/wiki/Chess#En_passant

"En passant" is a flanking move in (the rules of) war.

Can you get past your opponent without him or her knowing, or do you call the bluff?

If you are standing face-to-face with your opponent (your pawn on the 6th rank), then, you know that you and your opponent are face-to-face. Your opponent might attack, or, you might attack. Or, either of you might pass each other with a different objective in mind.

On the other hand, if you are in enemy territory, more than 1/2 way across the board, and, your opponent is standing-off in his or her territory, what if that opponent comes around the back of a hill and flanks you (runs past your troops)? If you "catch" your opponent doing this, you might "out-flank" and take. Otherwise, your enemy has advanced (you forfeit the right to attack, the same as ignoring two pawns threatening each other).