The native express ad format has been discontinued
In October 2017 we announced that native express would be turned off starting March 1st. All native express ad units have now stopped serving ads and have been removed from AdMob. Historical reporting data for these ad units will remain available.
If you had any active native express ad units, you can replace them with banner ad units. Alternatively, if you’re eligible for the AdMob native advanced beta program, we suggest you recreate your native ad units in that format.