How to detect when a animation is finished playing?

Or to add on the above answer, you can wait until it is done playing in the same block of code.

local animationTrack = humanoid:LoadAnimation(animationInstance)
animationTrack:Play()
wait(animationTrack.Length)
--finished playing

repeat wait() until not animationTrack.IsPlaying

If you have the animation track stored in a variable, you can use its Stopped event.

myAnimTrack.Stopped:Connect(function()
    -- do things
end)