[Mean]

Of course it said unknown command because you didn't add

pawn Код:

dcmd(giveallscore, 12, cmdtext);

to your OnPlayerCommandText.

This is necessary when using DCMD.
Click me for more info!

Also:

pawn Код:

dcmd_giveallscore(playerid,params[]) {
    if(AccInfo[playerid][Level] >= 3) {
        SendCommandToAdmins(playerid,"GiveAllScore");
        for(new i = 0; i < MAX_PLAYERS; i++) {
            SetPlayerScore(i, GetPlayerScore(i) + strval(params));
        }
        new string[128];
        format(string,sizeof string,"|- Administrator \"%s\" has given everyone %d score!", pName(playerid), strval(params));
        SendClientMessageToAll(blue, string);
    }
    else {
        ErrorMessages(playerid, 6);
    }
    return 1;
}

PS: What CrazyMax and Romel posted is SETallscore, not GIVEallscore.