Currently we’ve received an issue report from our Windows 10 Insider that when scanning their PC with Windows Defender, the error 0x8007139f happened.

Error 0x8007139f fixed!!!

For more details, the Windows Defender error message window said “The scan couldn’t be started.  The group or resource is not in the correct state to perform the requested operation.
This is a known issue that usually happen when the system files are damaged or corrupted.
If you get this problem, please try these steps to solve it:

1. Restart your PC
Many times the issue is resolved by simple restart.
2. Remove existing antivirus and antispyware software
If your PC still has another antivirus installed or if one was installed previously then you should use appropriate removal tool to remove all third party antivirus and antispyware programs. You can download removal tools from here:
3. Scan your PC for malwares
You can scan your PC with Microsoft Safety Scanner to ensure your PC is not infected.
4. SFC scan
System File Checker (SFC) tool repairs corruption in system files. Use this tool to verify whether Windows Defender is corrupted or not. Follow this KB article:
5. Clean Boot
Start your PC in clean boot status to ensure any 3rd party application is not conflicting with Windows Defender. Here is a support article that will help you:
6. Restart Security Center Service
As reported in this and this thread, restarting Security Center service can help in solving the problem. To restart Security Center service, follow these steps:
  1. Press Windows key + R. This will open Run. Alternatively, you can go to Start and search for ‘Run’.
  2. In Run dialog box, type ‘services.msc‘ and hit enter.
  3. In Services, search for ‘Security Center‘.
  4. Right click on ‘Security Center’ and click on ‘Restart‘.
7. Delete conflicting Registry Entry
Some malwares adds malicious entries in registry that blocks real antiviruses from running. To remove these entries, follow these steps:
  1. Press Windows key + R. This will open Run. Alternatively, you can go to Start and search for ‘Run’.
  2. In Run dialog box, type ‘regedit‘ and hit enter. This will open Registry Editor.
  3. In Registry Editor, navigate to HKEY_LOCAL_MACHINESoftwareMicrosoftWindows NTCurrentVersionImage File Execution Options.
  4. In this key, if you find any entry for MSASCui.exeMpCmdRun.exe or MsMpEng.exe then right click on it and click on Delete. If you do not find any of these entry then it is normal and you don’t need to do anything.
Update Nov 4th 2016:
Some Insiders said that the above solution does not work for in their case. Instead, they provide us some other method to fix the issue. Please take a try if above does not help.

Method 1
If you’re running any other security software, you may disable it temporarily and then try to start Windows Defender.
Disable anti-virus software
Disable antivirus software – Windows Help
Note  : Antivirus software can help protect your computer against viruses and other security threats. In most cases, you should not disable your antivirus software. If you do have to temporarily disable it to install other software, you should re-enable it as soon as you’re done. If you are connected to the Internet or a network during the time your antivirus software is disabled, your computer is vulnerable to attacks.

Method 2
Set the Windows Defender service to Automatic instead of Disabled or Manual. To do this, follow these steps:
a.Click Start, type services in the Start Search box, and then click Services in the Programs list.
 If you are prompted for an administrator password or for confirmation, type your password, or click Continue.
 b.Double-click the Windows Defender service, and then in the Startup type box, click Automatic.
 c.Click OK.

Method 3
If the procedure in Method 1 does not resolve this issue, follow the steps:
a.     Click Start, type Windows Defender in the Start Search box.
b.    Right click and select Run as administrator.