@see Where are the primary and secondary tab blocks?
You probably installed the minimal profile. And although these blocks are part of the Seven theme they only get installed in the standard profile. Sounds a like a bug. As they rather should get installed during the installation of the theme instead of during the profile installation.
Try this:
Use Drush or Drupal console or the built-in Configuration
Synchronisation to export your current configuration. Then copy the
config files...
core/profiles/standard/config/install/block.block.seven_local_actions.yml
core/profiles/standard/config/install/block.block.seven_page_title.yml
core/profiles/standard/config/install/block.block.seven_primary_local_tasks.yml
core/profiles/standard/config/install/block.block.seven_secondary_local_tasks.yml
... into your config folder - which is probably
/sites/default/files/config_[hashcode]/sync/. Then do a config import
(using one of those 3 tools above).