How to add block to a single page that is not included in the allowed content type list?
With Drupal 8 you can have multiple instances of a block.  So, the easiest way to have this block show on the article page would be to do the following:

In the structure->block layout page, go to the region you already have the block created.

Place a new block and select your description block again.

In the configuration settings, set the page restrictions to use:  /node/&lt;nid&gt; where  is the article you want the block to be displayed.  You can use aliases here but the nid is best because this supports alternate languages if you need them.

Note: I assume you already have a block instance set up limited to the other content types.  If not, repeat and use the content types restriction to limit it.