Unable to change the value of a variable within a Catalog Task
Release or Environment
In this case, the behavior experienced is due to a UI Policy which is setting the variable to read-only (a Client Script can cause the same kind of behavior). Hence, the value cannot be changed/edited.
If it is required that the variable value be modifiable, the user can either make the UI Policy active false or place some condition upon it whereby the variable is read-only to a select group of users and editable to another group of users - whatever best meets the business needs of the user.