A missing template will never result in a 404 error. That’s not how templates work. The reason you’re getting a 404 is that you’re attempting to access a URL that does not exist.
{Site URL}/{Taxonomy name}/ doesn’t exist. There’s nothing for WordPress to display at that URL. WordPress templates only ever display posts, but this URL doesn’t specify which posts to display. You need to specify which collection term to display, and then access posts with that term at {Site URL}/{Taxonomy name}/{Term slug}. All posts would be accessible at {Site URL}/{Post type name}/, but {Site URL}/{Taxonomy name}/ will return a 404.
If you want to display a list of terms within a taxonomy, then the only way to do that would be to create a page with a custom template that lists all terms. It’s not a native type of template in WordPress.