You have to let your session know that reading this information from a non-primary server is OK (literally)…
-bash-4.1$ mongo
rs1:SECONDARY> show dbs
2018-02-01T10:37:58.457-0500 E QUERY [thread1] Error: listDatabases failed:{
"operationTime" : Timestamp(1517499476, 1),
"ok" : 0,
"errmsg" : "not master and slaveOk=false",
"code" : 13435,
"codeName" : "NotMasterNoSlaveOk",
"$clusterTime" : {
"clusterTime" : Timestamp(1517499476, 1),
"signature" : {
"hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
"keyId" : NumberLong(0)
_getErrorWithCode@src/mongo/shell/utils.js:25:13
Mongo.prototype.getDBs@src/mongo/shell/mongo.js:65:1
shellHelper.show@src/mongo/shell/utils.js:813:19
shellHelper@src/mongo/shell/utils.js:703:15
@(shellhelp2):1:1
rs1:SECONDARY> rs.slaveOk()
rs1:SECONDARY> show dbs
admin 0.000GB
config 0.000GB
local 0.001GB
mongoengine_test 0.004GB
rs1:SECONDARY>
