Cannot implicitly convert type 'void' to 'string'

I think it very much clear, if you just see what you have done.

 

You have written a VOID function, which means, it returns nothing, and as you can see it is indeed not returning anything and you are trying to assign the return value of that function to a String variable then how do you exactly expect it to work.

 

What is the intension of your code if(product != null) ? Please clarify and / or modify your code accordingly.