The Full Error Message:
All error codes will be associated with some messages. The complete message that is related to the Device Manager error 21 is given below:

Windows is removing this device. (Code 21)
Cause of the error:

The error 21 indicates that Windows is actually in the process of removing the device (the one that is connected to your Windows computer). But this device is still not completely removed from your computer. So, the Device Manager error 21 is temporary. This means that it exists only when you are attempting to remove the device.

Troubleshooting Method:

There are two ways to fix the error. You can either wait for Windows to finish the process of removing the device entirely or restart your computer.

Wait patiently for some time.
After a couple of minutes, press the F5 key.
Pressing this key will update the Device Manager view.
In case the above steps fail to resolve the issue, try restarting your computer.

To do the same, these steps will be helpful:

Click the Start icon.
Next, click on Shut Down.
In the Shut Down Windows dialog box, choose the Restart option.
After you complete the above steps, your computer will restart.
Now, check if the Device Manager error 21 is fixed. By now, it should be.