How to find site errors? Are there resources that help you do this for free or at a minimal cost? I want to improve it

You must optimize your site for sure if you want it to work correctly. I use the website error checker https://pr-cy.io/ to find errors. It allows you to find and fix errors. The site has free and paid features. A quality audit of the site will improve the position of the site. I have seen many similar resources. I liked this one the most. I'm still using the free features of the site. I see the result.