Hello,
Thank you for contacting Rank Math. We are sorry for any inconvenience this must have caused you.
To be able to set the line height for your table of contents, go to the table of contents block settings, click the three dots at the right end, and select line height. Please see the screenshot below for reference.

If you can not see this setting in your Table of Content Block settings, then it means your theme does not support it.
You can use the following CSS code to style the line height for the table of contents in Rank Math:
.wp-block-rank-math-toc-block nav ul li {
line-height: 1.5;
}
You can add this code to your theme’s style.css by going to your Dashboard-> Appearance-> Customize
This code will set the line height of the table of contents items to 1.5 times the font size. You can adjust the value as per your preference.
We hope this helps you. Please feel free if you have any further questions.
Hello,
We are super happy that this resolved your issue. If you have any other questions in the future, know that we are here to help you.
If you don’t mind me asking, could you please leave us a review (if you haven’t already) on https://wordpress.org/support/plugin/seo-by-rank-math/reviews/#new-post about your overall experience with Rank Math? We appreciate your time and patience.
If you do have another question in the future, please feel free to create a new forum topic, and it will be our pleasure to assist you again.
Thank you.