On the workspace, select the form you wish to set in "No Production";
Lift the black arrow at the bottom of the card to see the menu;
Tap on the last option with the strikethrough plant icon in the button;
Type in your "No Production" reason;
Tap on Apply;
The form will be removed from your workspace and will be automatically marked as in No Production.
A form with this status will never be invalid, even though there might be empty mandatory fields in the form.
The form will be available in the archive and on the Paperless website, where you will be able to see all information about the No Production status of those forms.
See also how to set multiple forms in "No Production" from the Paperless website.