How to fix a broken or missing block?
You have created a Custom block and the custom block was placed in a region, then you lost the custom block maybe was deleted or you made a Config Export from one environment and a Config Import in other. So the custom block is no longer exists but in the Block layout page (admin/structure/block), the references exist.

So, remove the missing Block from the Block layout page (admin/structure/block) and create again the Custom block and place it again in the region.

There is a core issue open discussing this problem: https://drupal.org/project/drupal/issues/2756331

This can also be overcome by using this module: https://www.drupal.org/project/recreate_block_content