How can I see my seed in Minecraft?

The command "/ seed" no longer works and I would like to have the world in single player too

First a little counter question, which version are you playing on?

It sounds like you're playing on a server and want to play the world of those in single player.

That it does not work is probably due to the permissions on the server.
So ask an admin what seed the world has (if it is generated at all and not self-made)

If he says no, he will have his reasons.

1.15

@ Suddendeath555

I think it makes sense if you create a new folder for 1.15 and reinstall 1.15 there. You can easily transfer the game status so that you do not lose any progress.

What exactly happens when you type / seed?