It is hard to evaluate, when you have not posted, what changes have you done, but in general, I think you should rather extend the templates, then copy them. Please see Shopware 6: Extending a storefront block.
Are you sure, you have copied it correctly from vendor/shopware/storefront/Resources/views/storefront/layout/footer/footer.html.twig to your theme directory?
I have no such thing as category_url in my version of this file. Perhaps you are using some third party theme or plugin, that expects to have category_url method available?
In general, you can create links to categories like this: