No mobs are spawning in my mob farm

There are several reasons why mobs may not spawn in a farm.

• Mob spawning is set to false in the settings.
• You are the wrong distance from the point you want the mobs to spawn.
• The light level is too high (though judging from your screenshot this is not the case).
• The mobs are spawning elsewhere.
• There is not enough space for them to spawn / there are carpet or slabs preventing spawning (but again, not likely based on given info).

There is also a possibility that the tutorial you watched was meant for Bedrock Edition.

If you would like me to improve this answer I would need a screenshot of this farm during the day and possible the video you based it off of.