Lines 6-9 define a method called currencyConverter and it has a single parameter of type float called rupee. So when in line 2 you call the method you must pass a float variable or value.

You also had println(dollar); but dollar is a local variable declared in the currencyConverter method so is not available outside that method. Also the current exchange rate is ~§00 rupees to the US dollar so I have altered the formula accoringly

1. void setup() {
2.     float currency = currencyConverter(1200.5);
3.     println(currency);
4. }
5. 
   
6. float currencyConverter(float rupee){
7.     float dollar = rupee / 102;
8.     return dollar;
9. }