How to switch screen unlock from Pattern to Slide
Try this: 


Open your Settings.
Scroll down and select Security.
Clear Credentials.
Check the Screen Lock setting to see if the low-security options are now enabled.


If that doesn't work, you can revert your lockscreen security to Slide.


Set your lockscreen security to Pattern from Settings>Security>Screen Lock.
Click the power button on your phone to lock it.
Click the power button again, but don't unlock your phone. There should be a button labelled Forgot Password. Note: On some devices, you may need to incorrectly enter the password/pattern several times until that option appears.
Enter your Google account credentials.
Your lock settings should now be Slide. Note: With this method, the option for Slide security will still by disabled, but you should still have it as the lockscreen security on your phone.