Quién me da un pequeño empujoncito?

[Castillo]

addEventHandler ( "onClientGUIClient", mp5, mp55, false ) 

pusiste "mp5", pero tu variable es "MP5", recuerda que Lua es case-sensitive.

[FraN-724]

Intenta así

Client

function asd() 
        arma = guiCreateWindow(322, 130, 190, 227, "Arma", false) 
        guiWindowSetSizable(arma, false) 
  
        MP5 = guiCreateButton(29, 68, 139, 113, "MP5", false, arma)     
    
    showCursor (true) 
    addEventHandler ( "onClientGUIClick", MP5, mp55, false ) 
    end 
  
addCommandHandler ("mp5", asd) 
  
function mp55 () 
showCursor (true) 
triggerServerEvent ( "onGreeting", getLocalPlayer() ) 
end 

Server

function greetingHandler () 
    
 giveWeapon ( source, 29, 200, true) 
  
end 
addEvent( "onGreeting", true ) 
 addEventHandler( "onGreeting", getRootElement(), greetingHandler )