local node. I verified 6 confirmations before attempting masternode start

$ darkcoind walletpassphrase $P 60 ; darkcoind masternode start $P
masternode input must have at least 6 confirmations

$ darkcoind listreceivedbyaddress
[
{
"address" : "CENSORED",
"account" : "",
"amount" : 1000.00000000,
"confirmations" : 6,
"txids" : [
"CENSORED"
]
}
]