There may be many reasons for the QuickBooks Update Error 15106 to occur. These factors are:
1. Antivirus applications may be installed in your computer. This may be preventing the QuickBooks application to access the internet connection or the updation of the files.
2. There is a possibility that the administrator of the system may have restricted your access to certain files only. Hence you may have to login as an administrator to access the files.
3. There is a chance that a firewall or antivirus application is identifying the QB update program as a malware activity. In such cases, it will deny access to the updates.