Drupal 8 programmatical block can't be place in "Block Place UI"
Message have say


  Fatal error:  Class 'Drupal\Core\block\BlockBase' not found in
  /vagrant/src/d8/modules/custom/pnc/pnc_block_test/src/Plugin/Block/PncBlockTest.php
  on line 23


problem here is wrong typo block not Block

replace 

// Module block exit. But this class not exit
use Drupal\Core\block\BlockBase;


with

//path folder core/lib/Drupal/Core/Block
use Drupal\Core\Block\BlockBase;