How can I assign Single value to a variable dynamically?

Hi aforapple_27 

Try this instead:

Amount for Previous Year =
var currYear = SELECTEDVALUE('Raw data'[Year])
var prevYear = currYear - 1
return 
CALCULATE(SUM('Raw data'[Amount]), 'Raw data'[Year] = prevYear, ALL('Raw data'))  

This should return you the amount for previous year.