...
queue = []
...
elif message.content.startswith('!queue'):
queue.append(message.content.split()[1])
elif message.content.startswith('!play'):
server = message.guild
voice_channel = server.voice_client
player = await YTDLSource.from_url(queue[0], loop=client.loop)
voice_channel.play(player, after=lambda e: print('Player error: %s' % e) if e else None)
await message.channel.send(f'**Now playing:** {player.title}')
del (queue[0])
voice_channel.play(player, after=lambda e: print('Player error: %s' % e) if e else None)