[Threshold]

pawn Код:

CMD:getping(playerid, params[])
{
    if(!IsPlayerAdmin(playerid)) return SendClientMessage(playerid, -1, "SERVER: Unknown Command");
    new targetplayer;
    if(sscanf(params, "u", targetplayer)) return SendClientMessage(playerid, -1, "USAGE: /getping [ID]");
    if(!IsPlayerConnected(targetplayer) || targetplayer == INVALID_PLAYER_ID) return SendClientMessage(playerid, -1, "Player is not connected.");
    new str[25];
    format(str, sizeof(str), "Player's ping is %i.", GetPlayerPing(targetplayer));
    SendClientMessage(playerid, COLOR_LIGHTBLUE, str);
    return 1;
}

You should use the "u" parameter when dealing with player names and/or IDs. Read more about SSCANF specifiers here:
https://github.com/Y-Less/sscanf/wiki