I am unable to drag files into collections

Make sure you drag from inside the photo, not the border around the photo. And indeed be aware of the difference between collections, smart collections, and collection sets.

Do you have create normal collections, or are this smart collections or collections set(s)??

Smart collection are filled by the appropriate conditions and collections sets can't include images. The can include only other collections.

How to create photo collections in Lightroom Classic?

If this doesn't help try to reset the preferences of Lightroom Classic:  How to reset Lightroom Classic preferences (adobe.com)

It's recommended to backup your preferences before you reset the preferences to the default settings: 

https://helpx.adobe.com/lightroom-classic/kb/preference-file-and-other-file-locations.html

If this also doesn't help we need more informations about your issue and your system.

Please post a screenshot in which we can see your collections view in the Library.

Also provide a copy of your 'System Info'. This can be get from the LrC Help > System Info menu item. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.