As part of our continued efforts to streamline ad serving, we’ve made a change to creative asset requirements. As of June 28, 2021, Studio, Campaign Manager 360, and Display & Video 360 no longer support creatives that include more than one HTML file. Existing creatives with more than one HTML file can no longer serve and have been archived in both Campaign Manager 360 and Display & Video 360.
If your creative uses more than one HTML file, you'll need to consolidate them into a single HTML file or use CSS or JS files instead of additional HTML files.