This indicates that you're using the style/banner override feature, and you've chosen a plan that does not support this.
If you're using the style/banner override feature you will need to check the configuration for each page on your site, see this guide. This feature is available with the Unlimited plan. If you sign up for a package that does not include this feature it will be removed, and you will have the same banner and style across all site pages.
Note: Squarespace 5 is our legacy platform from 2012. Although we still support Squarespace 5, fixing bugs or issues is a lower priority for us while we focus on our current platform, Squarespace 7.