I know that there are already some threads about this issue. Here is the full error for anyone who's not familiar:

You don't have permission to mount the file.

As many others have pointed out the ISO file does indeed mount anyway. Based on that the error message appearing seems to be the real issue. I work in software and if an error message is being produced in error it is a bug. It is obviously just an annoyance since it does not affect the desired functionality of mounting the ISO file. I would like to know if this issue is being addressed by Microsoft or if there is a manual fix that will prevent the erroneous message from appearing.

if you have a SD card reader, change the drive letter (disk management MMC) to a high letter like W.

Surely someone has a "real" fix for this.  And, no Windows 8.1 does not mount the file, in spite of the error message.

I am trying to install

Microsoft 60-day Evaluation of Office Professional Plus 2013

I downloaded the file but I still get the error message

You don't have permission to mount the file

I have Windows 8.1

I've tried the Chat and Phone support and some can't help me because they don't know how and other don't want to help me.