After you run a gpo backup from gpmc and you receive the following warning: The security principal [S-1-5-21-2140059808-1731083156-2602769309-500] reference in extesion [Security] cannot be resolved................
Details:No mapping between account names and seucirty IDs was done.
Translation: Someone making changes to a group policy adds a group or user account and decides to delete the account from active directory and forgets to delete group or user account from the group policy. :x

I downloaded XML Notepad 07 and it gives a very organized view of the group policy file Backup.xml. I was able to locate the ID [S-1-5-21.....] and I verified that the user account had been deleted. So I went to the GPO in question located ID [S-1-5-21-....] removed it from the GPO. I reran a successful GPO backup using gpmc.