If you are unable to download The Elder Scrolls Online on your Playstation 4, please try the following:
Make sure you are logged in to the PlayStation Network account that initially purchased the content. Sign out of the PSN and back in to ensure you are using the right account.
Check that your PS4 has enough hard disk drive (HDD) space for download and installation.
Check that your PS4 has the most recent software installed. Check the system software version on your PS4 by going to Image (Settings), then System Software Update.
Check that the PSN is not undergoing any maintenance that might affect your ability to download content here.