Google Ads recommends using secure URLs (https instead of http). Keywords with a Final URL that don't meet this criteria will be flagged in the dashboard as shown below (under Audit Alerts/Keywords).
Click on the alert to see individual keywords together with their URLs as shown below. To solve the issue, simply select the keywords you want to fix (or select all of them) and click on the button Change keyword URL to secure one. This will replace the final URLs with their equivalent starting with https:// .