File with .dng extension cannot be opened in development mode in LR

DNGs from what source? An extension alone doesn't make it a DNG hence the question.

Try resetting the preferences of Lightroom Classic:  https://helpx.adobe.com/lightroom-classic/help/setting-preferences-lightroom.html

Backup your preferences prior to resetting the preferences: 

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

If this doesn't help, please give us more informations including the following :

Adobe product and version number

Operating system and version number

The full text of any error message (s)

What you were doing when the problem occurred

Screenshots of the problem and/or error messages

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut as that is just so much dead space to us non-Techs.