Your code worked for me, except it wouldn't delete unless I added await on message delete (message.message.delete() -> await message.message.delete()), but even if I didn't add await, I wouldn't get that type of error. The only problem why it wouldn't delete the message for you is because you didn't add await. You shouldn't be getting an error saying "type object 'Context' has no attribute 'message'" if I'm correct. Are you sure you're not getting the error from a different part of the code that isn't the one you gave? Or maybe while pputting the code in stackoverflow you removed the part where the error is coming from in the reg command? Or you made a change but never saved?
@client.command()
async def reg(message, oktaz):
await message.message.delete()