I was able to resolve this on my own but will post the solution in case someone else has this same issue. During the MSME upgrade from 8.5.1 to 8.6, I receive an error "Error 1303. The installer has insufficient privileges to access this directory: D:\Program Files(x86)\McAfee\MSME\Config. The installation cannot continue. Log on as an administrator or contact your system administrator." You then have 2 options, Retry or Cancel.
We are running 3 standalone instances between 3 Exchange servers but replicating the config between them. If you stop the "McAfee Security for Microsoft Exchange Replication Service", you can then click Retry on the error popup and the installation will successfully complete.