Comment on WordPress Widgets Tutorial by SEO Dave.
Possible causes are browser issue: try a different browser.
Conflicting plugin: disable plugins one by one and check if it works.
Other things to try are reboot your PC.
Under “Appearance” >> “Widgets” on the top right click “Screen Options” and if it says “Enable accessibility mode”, click it and it will say “Disable accessibility mode” and widgets can be edited another way.
If the above doesn’t help search Google for
WordPress Drag and Drop Widget not Working
There’s loads of WordPress users with the same issue, maybe one of the support answers will fix your issue.