Try this
https://social.technet.microsoft.com/Forums/en-US/f8aba9b6-2b18-4d84-8622-129d379d9032/upgrading-the-sccm-1802-to-1806-and-getting-failed?forum=ConfigMgrMigration
Issue about permissons ?
Check permission on folder C:\Program Files\Microsoft Configuration Manager.
In my case inheritance was disabled. I dont know why. Enabled it and check on box Replace perm....