mongodb set feature compatibility to 3.4 fail
You probably ran your command

 db.adminCommand ({getParameter: 1, featureCompatibilityVersion: 1})


on a mongos instance. The documentation states in this case:


  The operation is undefined on the mongos instances. For a sharded cluster that has access control enabled, to run the command against a member of the shard replica set, you must connect to the member as a shard local user.


When you connect to a mongod instead, the command should work as expected.