Why didn't this flush win the game?

The other player wins with a straight, but I had a flush. Why do I lose?

Seems like, you did not have a flush.

According to the Omaha Hi/Lo Poker rules from Pokerstars:

All players use exactly two cards from their four hole cards in conjunction with exactly three cards from the board to make the best five-card poker hand possible.

Since you only had 1 diamonds card (King) in your hole cards (your hand), you could not make a flush.

The rules for Omaha High Low state that you have to use two cards from your hole and three from the board to make a hand.

Your flush only included one card from your hole.