It seems that the regtest is not running.
How about adding regtest=1 in bitcoin.conf ?
If not try this method below.
It seems the bitcoind is the server starting up.Anyone else with the same problem you just need to open one more shell.
I used mingw.
First shell.
Use "bitcoind -regtest -daemon" to start the bitcoin server.
Leave the first shell open and open a second one also.
Use "bitcoin-cli -regtest" then space your "command"
example
"bitcoin-cli -regtest getblockcount"
to stop the first shell normally use this on the second shell.
"bitcoin-cli -regtest stop"