Resources for finding site errors

My site is not working properly. There are errors on the site. How to find them? What resources should be used to find bugs? Are these resources paid?

Resources for finding errors are mostly paid. I found one with some free features. This is website SEO Checker https://pr-cy.io/ I had few visitors, rankings worsened in search engines. The resource helped me find errors. Recommendations were given on how to do it. There are also paid features on the site. I am satisfied with the result of working with the site