Hello @alinakvita13,
Thank you for contacting the support.
The Free plugin has all the necessary features that you need to boost the on-page SEO performance of your site for a better ranking in the SERPs. Rank Math PRO is for people who wish to get additional features that we offer. PRO has enterprise-level features that assist with dominating your niche. https://rankmath.com/free-vs-pro/
Please note that ranking on Google depends on so much more than just your on-page optimization. You could have the perfect article and still struggle to break the top 10 in SERPs. Here are some of the factors Google considers when ranking you:
1. Your website’s authority
2. Your content’s quality and how better it is than the other article currently ranking.
3. How competitive the keyword is.
4. How many backlinks you have and if they are better than what the other guys ranking for that keyword have.
5. Your website’s crawlability.
6. Frequency of new content and content updates.
7. So much more that there is a dedicated blog post on it by Brian Dean ?? https://backlinko.com/google-ranking-factors
We would recommend that you keep at it and work on acquiring more links to break into the top SERPs.
Hope that helps and please do not hesitate to let us know if you need our assistance with anything.