This error might arise on the 20th day of the control period, when we start checking if the article is indexed in Google. All articles have to be, and if our check finds that yours isn’t, you’ll get this error. You need to get the article indexed in Google in order to solve the error and avoid the cancellation of the mission.
Use your Google Search Console to identify the indexing issue and request the indexation. You can find here our complete guide to indexing: https://getfluence.com/google-indexing-guide-ensure-page-visible-search-engine/.