One thing wrong. You need to put ".." (or "...") between the dates for a "range" find. Also, it is no longer necessary to use Insert Calculated Result in modern versions of FileMaker; it's not "wrong", but you can use the usual Set Field instead.
Set Field [ Activities::Date of Activity; Activities::Start Date & ".." & Activities::End Date ]
I am assuming that Activities::Start Date and Activities::End Date are global fields? They should be. They also should be named so you can tell by their names.
A couple of enhancements. If one of your IsEmpty tests fail, you could put a Go To Field before the Exit Script. That way they'll be left in the field and can enter a date easier.
Optionally, at the end, if you use Set Error Capture ["On"] (before the Perform Find), then test for Get (LastError) = 0 right after, then you could only clear the globals if a Find was successful; that way they could see the dates which did not work, and modify, without having to start over.
thanks for your suggestion that did provide the desired output. I'm looking for run the query and the report in the same layout. So I'll play with this a bit more to see if I can get it to work.