You need to pass true or false for the 'unlock' parameter e.g. to lock, set unlock to false.
To lock the tx output:
./multichain-cli demochain lockunspent false
'''[{"txid":"73d6d4b9c9a2f0af8850ce8e5efbd915817cf9d393f80d87ed54813474d1172f", "vout":0}]'''
To verify it is locked:
./multichain-cli demochain listlockunspent
[
{
"txid" : "73d6d4b9c9a2f0af8850ce8e5efbd915817cf9d393f80d87ed54813474d1172f",
"vout" : 0
}
]
And to unlock:
./multichain-cli demochain lockunspent true
'''[{"txid":"73d6d4b9c9a2f0af8850ce8e5efbd915817cf9d393f80d87ed54813474d1172f", "vout":0}]'''
