For chat commands you use serverCmdYourCommandNameHere
Get rid of this line: %client = player.getPlayerName();
For messaging everyone, you use messageAll('', "Your message here"); - Please note that that the first parameter uses single quotes
For concatenating strings (putting strings together), you use the @ symbol
Example:
messageAll('', "The player " @ %client.name @ " has been killed");
For concatenating AND adding a space you use SPC
Example:
messageAll('', "The player" SPC %client.name SPC "has been killed");