$$Start...$$End is incorrect syntax for an expression.
If you are using stored find criteria, Set a single variable to: $$Start & "..." & $$End and use it in your stored criteria.
or use the fields from which these two variable get their values in the above expression.
On the other hand, you can also create a script that enters find mode, uses set field to specify the search criteria and then a Constrain Found set step can constrain the found set. You could use $$Start & "..." & $$End as the second parameter in the second parameter of a set field step.
You may find this thread of scripted find examples helpful: Scripted Find Examples