db.copyDatabase Error:

"errmsg" : "Got invalid BSON from external server while reading from cursor
:: caused by :: InvalidBSON: Cannot use decimal BSON type when the featureCompatibilityVersion is 3.2.
See http://dochub.mongodb.org/core/3.4-feature-compatibility."

This is because the featureCompatibilityVersion version is not correct
1. Check the current featureCompatibilityVersion version and
execute the command line db.adminCommand({getParameter:1,featureCompatibilityVersion:1})
to return { “featureCompatibilityVersion” : “3.2”, “ok ” : 1 }
2. Modify the featureCompatibilityVersion version
Execute the command line db.adminCommand ({setFeatureCompatibilityVersion: “3.4”})
to return { “featureCompatibilityVersion” : “3.4”, “ok” :1 } , after the modification is successful,
execute db.copyDatabase and it will be ok