This indicates that you're either using the code injection or the style/banner override feature, and you've chosen a plan that does not support this.
If you're using the code injection feature you can access this from [Website Management > Structure > Website Settings > Code Injection].
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.
Both of these features are available with the Unlimited plan. If you sign up for a package that does not include these features they will be removed.
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.