Hi @aimeeyin,
While the plugin is able to add code to those pages it really depends on the template of your category pages in order to add H1 tags to the right place.
If your pages already have a title you can try replacing that but if you don’t have a title on the page your only option is to use the “Site Wide Body” action but that will add the title at the very top of the page.
So this function does not for free plan, right?
It’s not related to the plan. In any version of the WPCode plugin your theme template can influence where you can add snippets.
In the scenario you described, if your theme doesn’t have an action to hook into that is relevant for adding the H1 tag on the page you can’t add the code there without making changes to the template.