As you are using NAPA, I presume you want to access host web list from app. Apps execute in different domain (called app web) than host web.
The sample code is
var context = new SP.ClientContext(appWebUrl);
var appContextSite = new SP.AppContextSite(context, hostWebUrl);
var list = appContextSite.get_web().get_lists().getByTitle('SitesList');
hostWebUrl from query string parameters
For more details, please visit this link.
To access host web data from app web, we don't need SP.RequestExecutor.js. Therefore, updated the code above.
Check this link for more info. According to this link:
By default, a SharePoint-hosted app is allowed to issue cross-domain calls to the host web, provided that it has proper permissions.