how to make entity tamed as soon as it spawns

What trigger are you using, It looks like EventTarget entity is the player and you’re spawning the Friendly Silverfish at the exact same location as the player. If so, you’re trying to tame the player and saying that the player is being tamed by a “source entity” (probably nothing). You need to make the friendly silverfish the one being tamed and make sure it’s by the player (the trigger should have either “source entity” OR “event target entity”).