LmtPrice = LastValue(C,1); //Returns constant number
PlaceOrder functions accepts only Strings/Constant Numbers.But in AmiBroker most of functions (99%) returns array and all price variables (O,H,L,C,V,OI, etc) are array only. You can check the AmiBroker help doc to see what a function returns.
A simple way to understand Array and Constant is
Teacher says Hello Students, here the students is array & it is pointing to all.
Teacher says Student Bharath scored 95% , here the student refers to constant i.e. it refers to the single person 'Bharath'.
Same way in AmiBroker 'C' or 'Close' refers to all the close values of a Bar. If AmiBroker has 1000 candles in a stock, the 'C' refers to all the close price of the candle. So 'C' is array of close price.
But the LastValue(C,1) always refers to the very last close price of the candle. It refers to single value, because only one value can be last.