[Konstantinos]

You need to declare it first and even if you do, it's null (empty). You'll need to format it to your needs.

pawn Код:

public OnPlayerConnect(playerid) 
{ 
    new
        string[ 32 ],
        _name[ MAX_PLAYER_NAME ]
    ;
    GetPlayerName( playerid, _name, MAX_PLAYER_NAME );
    format( string, sizeof( string ), "THE PATH GOES HERE", _name ); // CHANGE IT.
    if(fexist(string)) 
    { 
        gPlayerAccount[playerid] = 1; 
    } 
    else 
    { 
        gPlayerAccount[playerid] = 0; 
    } 
    return 1; 
}