Q:

Run-time error 9 in vba

Good day, I'm pretty much new in VBA, and I have a problem with path specifying, consider the following:
x = Workbooks(ThisWorkbook.Path & "/" & "06-17.xlsx").Worksheets(1).Cells(2, 1).Value

With this piece of code I have run-time error 9, but if I use the following code it works fine:
Workbooks.Open (ThisWorkbook.Path & "/" & "06-17.xlsx")
x = ActiveWorkbook.Worksheets(1).Cells(2, 1).Value

I have founded that run-time error 9 means that elements of arrays and members of collections can only be accessed within their defined ranges, but how it relates to my case, I have no idea. 
Why it happens?

A:

The index to the Workbooks collection is the file name (or an integer number), not the file path and name.
So Workbooks(ThisWorkbook.Path & "/" & "06-17.xlsx") would not be valid (because it includes the path), but Workbooks("06-17.xlsx") will be valid.
This is the reason why (or, at least, related to the reason why) you can't have two workbooks open in Excel at the same time if they have the same file name, even if they have been opened from two different directories.

