You should only mark your artwork as 'Unavailable' if it's not available for purchase. Maximum number of days you can mark an artwork as 'Unavailable' for is 60 days. If your work is not available for longer, you can mark it as sold or move it to drafts and publish later.
If your artwork available for purchase after a certain day/date, you can mark it as 'Available with delayed dispatch'.
To mark an artwork as 'Unavailable' -
Go to 'My Artworks'
Search/find the artwork you'd like to make unavailable
Click on 'Edit'
Scroll down to 'Availability' section
Choose 'Unavailable' from the drop down menu
Choose the date it's available from
You can choose to leave a note/reason for unavailability
Scroll all the way down and 'Update'
This artwork will now be shown as 'Enquire Now' and will not be included in any of our marketing and advertising campaigns.