By default, Yoast SEO views the title of the post/page as the first and only H1 heading you should have. Aside from the title of the post/page, are you adding an additional H1 in the content of the page? This sometimes happens with page builders and, as long as you are certain there is only one H1, you can ignore the message.
If you’re not sure or not using a page builder, this may be a plugin or theme conflict. Please disable all plugins except Yoast SEO, and use one of the default (Twenty*) themes. If the problem goes away, enable plugins one by one to identify the source of your troubles.
If you can install plugins, the Health Check & Troubleshooting plugin offers a troubleshooting mode that will disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site. Once in the troubleshooting mode, please activate only Yoast SEO and attempt to reproduce the issue you described.
If you are unfamiliar with checking for conflicts, we’d like to point you to a step-by-step guide that will walk you through the process: How to check for plugin conflicts
