You receive this message due to not having a hero of the matched element in your party. For example if you match 3 blue gems but do not have a blue hero, you will receive the hero missing message.

Tiles of a certain color will only cause damage if you have a hero of corresponding color in your team. For instance, if you don't have a blue hero in your team, all blue connections on the board will hit with the 'hero missing' message, and cause damage of 1.