No block to display at block settings in any contributed theme I am trying to use Simplenews module for newsletter, but there is no block to display at block setting in any contributed theme after enabling the module.

However it's show when I activate the core theme (Bartik). 

My problem is if I use contributed theme, there is no block of Simplenews module for enable the block on the front-end (on the block setting page).

What should I do in order the block is displayed?
Go to the Block Layout page at /admin/structure/block, then Place a Block in the selected region and select Simplenews subscription. Once saved, the block should be available.

Source: Simplenews on Drupal 8 - subscription block.