The native express ad format is being discontinued. Starting March 1, 2018, we will begin turning off ad serving and removing native express ad units automatically from AdMob.
If you’re currently using native ads express, make sure to convert your native express ad units and campaigns to banner ads before March 1, 2018. Following that date, only historical reporting data will continue to be available for native express.
If you’re eligible for the AdMob native advanced limited-access beta, consider switching to that format. The native advanced ad format is not yet generally available.