First, make sure that the embedding destination supports embedding iframe tags.
If you cannot embed iframe tags in a service that allows you to edit code independently such as WordPress, please check the user permissions.
Also, check the protocol (http/https) of the address of the page to which you want to load specified in the iframe tag. If the protocol of the page embedded with the customer's page and the iframe tag is different, it cannot be displayed.
Check the protocol (red text portion) of the address of the page to which you want to read specified in the iframe tag.
<iframe src="http://sample.com/sampel.html" 〜></iframe>
<iframe src="https://sample.com/sampel.html" 〜></iframe>