[Kane]

What...

ShowUsageMessage isn't a native SA-MP function. Sys and Dayrion both pointed out your mistake.

Your initial problem was with SendClientMessage().

Код:

if(isnull(params)) return SendClientMessage(playerid,"/oban [playername]");

Is incorrect.

Код:

if(isnull(params)) return SendClientMessage(playerid, -1, "/oban [playername]");

Is correct. -1 is where the color of the message goes.