On a Drupal 8 site, I see an error on a block which says:
This block is broken or missing. You may be missing content or you might need to enable the original module.
How can I resolve this error?
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.
The placement of the block is stored as configuration. This is what gets exported and synced between environments. The actual content of the block is considered exactly that: content. This means that when you import your new configuration into a different environment (or delete the block and then re-import the configuration) there is a mismatch between the configuration for the block and the actual block instance (which no longer exists).
To resolve this issue, you simply need to delete the configuration for the missing block and then recreate the block. This allows your site to resolve the discrepancies.
Another option would be to use the fixed_block_content contributed module to manage this process for you.
There is a Drupal Core issue discussing this very thing. https://drupal.org/project/drupal/issues/2756331