ndb_desc connection error (7.6.7) (no replies)

I am testing out NDB Cluster 7.6.7.

Just couldn't get ndb_desc work.

First I made sure that I have a free mysqld API NODE as node 6
[mysqld(API)] 3 node(s)
id=6 (not connected, accepting connect from any host)
id=53 @10.12.176.251 (mysql-5.7.23 ndb-7.6.7)
id=54 @10.12.2.135 (mysql-5.7.23 ndb-7.6.7)

If I run ndb_desc without --ndb_nodeid, I get error message like
ndb_desc -c 10.12.176.251:1186 sbtest1 -d sbtest -p
Unable to connect to management server.
NDBT_ProgramExit: 1 - Failed

Server log shows:
2018-08-03 18:31:57 [MgmtSrvr] WARNING -- Failed to allocate nodeid for API at 10.12.176.251. Returned error: 'No free node id found for mysqld(API).'

If I specify nodeid as:
ndb_desc -c 10.12.176.251:1186 --ndb_nodeid=6 sbtest1 -d sbtest -p
Unable to connect to management server.
NDBT_ProgramExit: 1 - Failed

Server log shows:
2018-08-03 18:41:24 [MgmtSrvr] WARNING -- Failed to allocate nodeid for API at 10.12.176.251. Returned error: 'Id 6 already allocated by another node.'

I am running it on the management server

Could anybody shed some light on this?

Thanks!