Geoff has given you the answer, but just in case you are wondering why, I'm betting that you knocked up a quick dummy formula based on a test spreadsheet, and then edited to get what you wanted on your real one.
What has happened is that your test spreadhseet has no spaces in either the filename or the tab name, and as such no quotes are required in the reference. In your real spreadsheet however, you have spaces in the filename, and as such require single quotes to show Excel where the names begin and end.
Because you introduced spaces to a name that was not surrounded by quotes, Excel now has no clue as to what you are trying to do, and so errors out on you. As soon as you fix that by using quotes, as Geoff has shown you, all should work fine.
Regards
Ken..............
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