I wnted to AFK at my farm with the /player attack but when I wrote /player there wasn't the command.

If you are playing on a server, make sure server has installed carpet

use /carpet commandPlayer to make sure the command is enabled (or if it's set to ops, make sure you're OP'd)

The solutions above are correct, you likely either not have enough permissions to run it (you can change the permissions with the commandPlayer rule), or carpet isn't on the server.