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

Question

I have been using drupal examples https://www.drupal.org/project/examples and developing my own examples for practice. But I have come against an issue I can't workout. Below is my link to the block code I have been developing.

d8 module block example on github

I can get the block to display under admin/structure/block, but when I press the "Display Block" button in the overlay I get no response can someone tell me why? See image

The error I am getting from the ajax call is

ajax.js?v=8.1.8:965 Uncaught AjaxError: 
An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: /d8/admin/structure/block/add/pnc_block_test/pnc?region=-1
StatusText: OK
ResponseText: 
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

What I have trying to achieve is the following

Answer 1

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;