Hi Poonam,
It is how you are defining your user variable. The way this form is configured is to support the use of expressions with other variables if you wanted. You put the expression/value you want in the expression box and the value box is the value your expression evaluates to. So let us assume you want a lattice constant of 0.4 um then for your case
Name: a
Expression: 0.4
Value: 0.4 (This is generated by the product)
To explain what I mean if you had a variable b that was 0.3 and you wanted your lattice constant to be 2 times that then you would do this
Name: a
Expression: 2*b
Value: 0.6
Hope this helps!