I tried to correct the code , check it out , i added the first part of it. 
function RobMarker(thePlayer) 
local pos = 281.12664794922 ,-1768.4904785156 ,4.5230431556702  
local theMarker = createMarker ( pos, "checkpoint" ,4.0 ,255 ,0 ,0 ) 
createBlip ( pos ,32 ,2 ,255 ,0 ,0 ,255 ,0 ) 
setTimer (RobMarker , 600000 ,0 ) 
end 
addEventHandler ("onClientResourceStart" ,getRootElement() ,RobMarker) 
  
function HitMarker(thePlayer,theMarker ) 
local thePlayer = getLocalPlayer() 
local markercheck = isElementWithinMarker(thePlayer ,theMarker ) 
 if ( markercheck ) then 
 outputChatBox ("keep inside the Marker and type /robtime to know what's the remaining time" ,thePlayer ,0 ,250 ,0 ,false ) 
 dxDrawText("ROBBERY IS PROGRESSING ( 50000$ )", 279, 143, 661, 201, tocolor(255, 0, 0, 255), 2.00, "pricedown", "left", "top", false, false, true, false, false) 
 setTime ( 00 , 00 ) 
 else return 
 end 
end 
addEventHandler("onClientMarkerHit" ,getRootElement() ,HitMarker )