How to add block to "add/edit" content page?
Here's how to display a block on node add/edit pages in Drupal 8:

Go to Structure -&gt; Blocks.  Select the block layout page for the theme you use to add/edit nodes. This is usually the admin theme, but it may also be the standard theme for your site if you want to display the block to non-admin users. If you want to show the block to both admin and non-admin users, you may need to configure the block settings (as described below) for both the admin theme and the standard theme.

On the block layout page, click Edit on the block you want to display.

Under Visibility, click Pages.

In the text field, enter:

/node/add/*
/node/*/edit


For Drupal 7, don't use the first slash:

node/add/*
node/*/edit


The * is a wildcard that says "any content/value can go here."

Then check the radio button for Show for the listed pages.

If you also want to limit the block to only show for certain content types, you can use the Content types option above Pages.