VBA in Excel: define file to open (I retrieve data from 2 Excel files)

Hello,

I have 2 Excel files open, file (A) is a data source file and file (B) is a file with various sheets being created with VBA.
My VBA needs to be clear which of both files it refers to when reading data, and I believe that is my problem.

I have the following written:

DataSourceFile = Range("LastFileEdited")
Set W2bk = Workbooks (DataSourceFile)        'I receive error 9: suscript out of range
Workbooks(W2bk).Open

How should I correct my VBA so that it opens the specified file? And if the file is already open, to select it?
Thank you for your help.