Ah...this is all done by Moodle Magic!
If you have access to Essential/config.php you will need to change this setting...
https://github.com/gjb2048/moodle-theme_essential/blob/master/config.php#L81
in order for the Add Block to work as it used to do in the Frontpage...
If you change that highlighted line to look like this...
'defaultregion' => 'hidden-dock',
then save that change, you should be able to see the ADD BLOCKS block in that hidden Admin section.
Cheers
Mary