[JordanDoughty]

pawn Код:

CMD:pm(playerid, params[])
{
    new str[128], str2[128], id, Name1[MAX_PLAYER_NAME] , Name2[MAX_PLAYER_NAME];
    if(sscanf(params, "us[128]", id, str2))
    {
        SendClientMessage(playerid, 0xFF0000FF, "USAGE:/pm <id/name> <Message>");
        return 1;
    }
    if(!IsPlayerConnected(id)) return SendClientMessage(playerid, 0xFF0000FF, "ERROR: This Player Is Not Connected.");
    if(playerid != id) return SendClientMessage(playerid, 0xFF0000FF, "ERROR: You Cannot Send PM Message To Your Self.");
    {
        GetPlayerName(playerid, Name1, sizeof(Name1));
        GetPlayerName(id, Name2, sizeof(Name2));
        format(str, sizeof(str), "PM To %s (ID: %d): %s", Name2, id, str2);
        format(str, sizeof(str), "PM From %s (ID %d): %s", Name1, playerid, str2);
        SendClientMessage(id, 0xFF0000FF, str);
    }
    return 1;
}

Compiled this with no errors, give it a go.

EDIT: you had 2 str's defined and some typo's.