Google Ads recommends using secure URLs (https instead of http). Ads with a Final URL or a Mobile FInal URL that don't meet these criteria will be flagged in the dashboard as shown below (see the Audit Alerts / Ads tab).
To fix this issue, click on the alert. You will see the list of ads with unsecured URLs along with their performance metrics. You can select the ads you want to fix and either pause or delete them by hitting the corresponding button. A copy of the affected ad(s) will be created automatically with the secure URL (using https://).