i am not being able to use multi window in my samsung s9 plus. plz help me to solve this

i have bought 2 days back s9 plus. why is it happening?? plz sew video...<script>LITHIUM.OoyalaPlayer.addVideo('https:\/\/player.ooyala.com\/static\/v4\/production\/', 'lia-vid-RvN2s0aDE6Z6OCf6VVJenoT4tRg3OKlJw320h240r731', 'RvN2s0aDE6Z6OCf6VVJenoT4t...