CreeperHunter, are you able to summon cows or entities in any other badges? That one is just a quiz, I believe. You'll begin creating mods that summon entities starting in the Functions badges.
That's strange that the artifact vox-l game isn't summoning entities - most of the time it works. Would take a screenshot of the code you ran in the downloadable vox-l game?
What type of browser are you using, and what version number, and what device or computer are you using?