Try this taxi code

CMD:taxi(playerid, input[])
{
    new tmp[30], id;
    if(!sscanf(input, "'fare'U(-1)", id)) {
        printf("ID: %d", id);
        SendClientMessage(playerid, color_grey, "Usage: /taxi fare [playerid]");
        if(id == INVALID_PLAYER_ID) return SendClientMessage(playerid, color_orange, "Invalid playerid");
        new str[128];
        format(str, sizeof(str), "You added the taxi fare onto %s (id: %d)", SendName(id), id);
        SendClientMessage(playerid, color_grey, str);
        SendClientMessage(playerid, color_grey, "You wrote 'fare'"); }
    else if (!sscanf(input, "'duty'U(-1)", id)) {
        SendClientMessage(playerid, color_grey, "You wrote 'duty'");}
    else {
        SendClientMessage(playerid, color_orange, "Unknown taxi action");
        SendClientMessage(playerid, color_grey, "Available commands: /taxi [duty/fare]"); }
    return 1;
}