You probably could Google that error message and get the basis of the problem pretty quickly. "Subscript out of range" errors invariable mean: "Some object (sheet, workbook, named range) that you gave me the name for to use.... Excel can't find that object."
The line of code supports that. MyFile is a string you got from J3. What exactly is that string? If even 1 character varies from the exact name of an already open workbook, it won't work.
Also, if the name is exact, is it already open? I'd recommend you set a workbook variable instead of a string, because then you can address the workbook from then on without using the real name again, just the wb variable.
Also, once you have the wb variable, you don't have to "activate" or "select" the objects anymore, you can just send data directly to the objects.
Here's as much of that macro as I could decipher to get you started rewriting it addressing those objects directly:
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