How can you enable friendly fire amx counter strike

You either have to be a server administrator or it will be programmed into the server beforehand.
If you have administrative privileges, you can change the cvar mp_friendlyfire to 1. You can also access the admin menu by entering "amxmodmenu" into the console.

If ,by any chance you're not admin and you mixed it up, and you are Rcon ... then type- rcon mp_friendlyfire 1