If night audit button is grayed out, It means the particular user does not have the privilege/rights to perform night audit. 

In order to assign privilege for night audit process, please refer below mentioned steps:

Path: Select Configuration(Login as Admin) >> User (button) >> Select the particular User >> Edit >> Operational privilege >> Check mark on night audit option >> Save.

Screenshot 1:

Restart eZee Frontdesk to apply the changes and check the same.