The problem is being caused by the & symbol present in the iframe tag's src link. You will need to escape it by replacing it with & to make the iframe tag get accepted by Blogger's XML parser. Also switching to HTTPS protocol will make sure that mixed content error doesn't occur
<iframe height='100' src='https://www.theworldcounts.com/counters/ocean_ecosystem_facts/coral_reef_destruction_facts/embed?background_color=white&color=black&font_family=%22Helvetica+Neue%22%2C+Arial%2C+sans-serif&font_size=14&token=ccfd8c44a4686cada3a2e343f1d56a9b98ac49d4' style='border: none' width='300'></iframe>