Try:
.Formula = "='" & strProjectName & "'!" & Cells(2, 7).Address
If your worksheet name (strProjectName) has spaces, you need to include the single quotes in the formula string.
If this does not resolve it, please provide more information about the specific error or failure.
Update
In comments you indicate you're replacing spaces with underscores. Perhaps you are doing something like:
strProjectName = Replace(strProjectName," ", "_")
But if you're not also pushing that change to the Worksheet.Name property, you can expect these to happen:
#REF errorThe reason for both is that you are passing a reference to a worksheet that doesn't exist, which is why you get the #REF error. The file dialog is an attempt to let you correct that reference, by pointing to a file wherein that sheet name does exist. When you cancel out, the #REF error is expected.
So you need to do:
Worksheets(strProjectName).Name = Replace(strProjectName," ", "_")
strProjectName = Replace(strProjectName," ", "_")
Then, your formula should work.