Trying to create a block plugin module
Ok can you go to drupal8/admin/modules 

and see :



Your: 


walker.info.yml is correct 
your Walker.module id fine (however i would use (walker as the dir and walker.module aka non caps)
your block looks fine 


Thinking Rename all to 
./modules/walker/walker.module
./modules/walker/src/Plugin/Block/WalkerBlock.php
./modules/walker/walker.info.yml

as could be Name space issue as "Drupal\walker\Plugin\Block" vs "Drupal\Walker\Plugin\Block" ?

but really every thing looks fine
When you install the module you should be able to place the block in 
admin/structure/block