Fix The selected task "{0}" no longer exists on Windows

This post helps Fix "The selected task {0} no longer exists" when running Task Scheduler. Learn more?

solution 1:
Registry Disclaimer: The steps mentioned below includes system registry modification. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs.
Click on Start; in the start search type regedit.exe and press Enter.
If you get a consent prompt, click on Yes.
Browse through the following location@ HKLM\SOFTWARE\Microsoft\Windows NT\Current Version\Schedule key and then delete ALL the subkeys in the folder above.
Save the changes you made and then exit Windows Registry Editor.

Solution 2:
Browse to C:\Windows\System32\Tasks\.
Rename the file ScheduledDefrag to ScheduledDefrag.xml.
Right-click the renamed file then select "Open With"
Note: If Notepad is in the drop down list, click it. Or, click on "Choose default program" then select Notepad from the list of programs and click OK.
Scroll down to the line <UseUnifiedSchedulingEngine>, and replace the word True with False on that line.
Save the file ScheduledDefrag.xml.
Start Task Scheduler.
Browse to Task Scheduler Library/Microsoft/Windows/in the left-hand pane.
Click "Import Task..." in the right-hand pane.
Browse to the ScheduledDefrag.xml file you modified earlier.
Click OK in the Create Task window.

Solution 3 (works for Windows 7): disable & re-enable the Disk Defrag Schedule service later.

Solution 4:
Open Windows Explorer and navigate to the tasks file folder (%SystemFolder%\Tasks\Microsoft\Windows) and find the folder which corresponds to the folder in which you received the error.
For some tasks you will be able to determine which files need to be deleted by comparing the list in the Task Scheduler with the list of files in Explorer.
See how to "Delete a Task" @ goo.gl/7sS37k (Applies To: Windows 7, Windows Server 2008 R2, Windows Server 2012 & Windows Vista).
You can also try the following 2 methods as well:
Method 1:
head to \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tree\.
Note down all the IDs in that folder.
Manually delete the GUIDs from the following locations:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Boot
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Logon
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Maintenance
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Plain
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks
Now, head to C:\Windows\System32\Tasks and erase the same items you found in Windows Registry Editor.
Method 2:
2.1 go to C:\Windows\System32\Tasks and see what items had been created.
2.2 delete any extra or different items from the following 2 locations in Registry Editor:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks &
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tree

Last but not least, see additional troubleshooting posts@ goo.gl/pzqWPL.