You need to post more details as usual. Nobody here can read minds. screenshots help. or any specific details.
In the top right corner is a small triangle facing to the right. This is, as usual, the play-button. It will open a new window (remember to allow pop-ups) in which you can play your level (which you lose after a few seconds because one of the archers dies).
Try changing “humans” to “Hero Placeholder” in the save-thangs goal in the settings:
Then only your hero has to survive, and it doesn’t matter if other humans die.