How to solve : run-time error 91 object variable or with block variable not set

Hi All,

In the attached file, whenever a cell under column D of DM PR Availability sheet is selected as Do Not Disturb, its timestamp is captured in the sheet DND timestamps.

All was working perfectly. However, all of a sudden, when i try to select Do Not Disturb now, excel shows below error:

run-time error 91 object variable or with block variable not set

Can someone guide and resolve the issue.

Thanks in advance.