@Yamsanibhavani 

While updating the record the dictionary override should not come into picture.

what error it throws while updating that field?

Did you try disabling that dictionary override and see if that's causing the issue?

Any data policy is blocking the update?

Any before business is blocking or clearing that field?

Did you try to update the record using background script?

If my response helped please mark it correct and close the thread so that it benefits future readers.