unable to create connection from the client(python)

simple default biucket( not password protected)

cluster = Cluster('couchbase://172.23.106.88', bucket_class=Bucket)

cluster.authenticate(ClassicAuthenticator(buckets={'default': ''}))

cluster.open_bucket('default')

Bucket('couchbase://172.23.106.88/default', password = None)
{_AuthError_0x2 (generated, catch AuthError)}<RC=0x2[Authentication failed. You may have provided an invalid username/password combination], There was a problem while trying to send/receive your request over the network. This may be a result of a bad network or a misconfigured client or server, C Source=(src/bucket.c,785)>

or with old approach 

Bucket('couchbase://172.23.106.88/default?select_bucket=true', password = None) 

{NotSupportedError}

<RC=0x13[Operation not supported], Couldn't create instance. Either bad credentials/hosts/bucket names were passed, or there was an internal error in creating the object, C Source=(src/bucket.c,737)>