Discord 'on_member_join' function not working

Solution 1:

You're probably using discord python 1.5.0 or above, it's a common error, you just need Intents. If there's a error, you are supposed to read it, it will redirect you to your bot in the discord dev portal, there you can activate the privileged gateway intents check this out

And add this to your code

intents = discord.Intents.default()
intents.members = True

client = commands.Bot(command_prefix='.', intents=intents)