The Classic Editor
Marks are available in the default Visual editor if you are using the Classic Editor plugin. If you are using the text or HTML editor, please switch to the Visual editor. You may need to update the content or refresh the page to activate the marks.
The text or HTML editor looks like this:
Switch to the visual editor which looks like this:
Gutenberg
In same cases, you may see a block that appears as the classic editor as shown in the image below
You will need to convert this block to a Gutenberg Block by clicking on the three vertical dots to the right and then selecting Convert. As shown in the image below. When you do that, the block will change to Gutenberg text blocks. Yoast is then expected to highlight these.
Pagebuilders
Please note that some features, like the readability markers, may be disabled when using page builders.
Related: Is The Plugin Compatible with Page Builders?