[thefirestate]

Why are you checking inputtext and frequency in the command which would make it more like /setfreq [FREQ] [FREQ]((Or whatever is inputtext used for)) However the error is because it should be like that:

if(!sscanf(params, "i", frequency))
{
/BLABLBALBALBLA
}
else return SendClientMessage(playerid, -1, "USAGE: /setfreq [FREQ]");

or you can use if(sscanf(params, "i", frequency)) return SendClientMessage(playerid, -1, "USAGE: /setfreq [FREQ]");