public OnPlayerConnect(playerid)
{
    TogglePlayerSpectating(playerid, true);
    new query[128], playername[MAX_PLAYER_NAME];

    GetPlayerName(playerid, playername, sizeof(playername));
    mysql_format(mysql, query, sizeof(query), "SELECT `Password`, `ID` FROM `accounts` `Name` = '%e' LIMIT 1", playername);
    mysql_tquery(mysql, query, "OnAccountCheck", "i", playerid);
    
    return 1;
}


forward OnAccountCheck(playerid);
public OnAccountCheck(playerid)
{
print("check");
    new rows, fields;
    cache_get_data(rows, fields, mysql);

    if(rows)
    {
        cache_get_field_content(0, "Password", Player[playerid][Password], mysql, 129);
        Player[playerid][ID] = cache_get_field_content_int(0, "ID");
        ShowPlayerDialog(playerid, Log, DIALOG_STYLE_INPUT, "Login", "Welcome player!\nYour account has been found in our database. Please fill in your password:", "Login", "Quit");
	}
    else
    {
      ShowPlayerDialog(playerid, Reg, DIALOG_STYLE_INPUT, "Register", "Welcome player!\nYour account has not been registered yet. Please fill in your desired password:", "Register", "Quit");
    }
    return true;
}

check the manual that corresponds to your MySQL server version for the right syntax to use near '`Name`'