I have been using a solution for creating journal entries. I create the entries, I then export to a text file in a folder that I can grab the data for an upload to the accounting software. For the last 2+ years, it has worked flawlessly. I set up a new computer (same versions of Win10 and FileMaker). Same file structure. When I tried to write the text file to the folder I get:
In the past, I have only gotten this message when the text file name was open on my computer. There are several texts files I create during the month. The new overwrites the old. The journal I am doing now, is a two line entry. Next week, there will be a couple that are hundreds of lines long; so I really need to resolve. 
The Samples folder has read/write permissions. But, in looking back to my old computer, the permissions were read-only.
Have you checked the file path?
All it takes is one detail different in the name of one of those enclosing folders and your file path is invalid. You then get that very uninformative error message.
It wouldn't be my preference to write such data to the program files folder and that folder is often write protected. When it is, you shouldn't be able to export data to that folder so your last sentence there is surprising.
One way to double check a file path is to place a file in that location and then manually insert the file into a container field using "store only a reference" as your insert option. You can then use getastext ( container field here ) to see the file path to this file and you can then compare that file path letter by letter to what you have specified.