#Value Error Still Not Solved

Hello I am new and need some help please

I have a sheet say cell A1 and I wish to populate cell a20 with the value in cell a1 which will do some simple math cell e1 will be a20 x b20 however if there is no value in a1 than it comes up with value error so how can I fix that. Isnt is some kind of if = than that value else zero