System environment:
win10 + virtualbox5.0.24
Docker,docker-machine version

JYC103@Fanne MINGW64 ~
 $  docker-machine.exe -version
 docker-machine.exe version 0.7.0, build a650a40
 
 JYC103@Fanne MINGW64 ~
 $ docker version
 Client:
 Version:      1.12.0
 API version:  1.24
 Go version:   go1.6.3
 Git commit:   8eab29e
 Built:        Thu Jul 28 23:54:00 2016
 OS/Arch:      windows/amd64
 An error occurred trying to connect: Get http://%2F%2F.%2Fpipe%2Fdocker_engine/v1.24/version: open //./pipe/docker_engine: The system cannot find the file specified.

There is now a docker host on docker-machine

$  docker-machine.exe ls
 NAME            ACTIVE   DRIVER      STATE     URL                         SWARM   DOCKER    ERRORS
 docker.20.127   -        none        Running   tcp://  192.168.20.127:2375           Unknown   Unable to query docker version: Unable to read TLS config: open C:\Users\JYC103\.docker\machine\machines\  docker.20.127\server.pem: The system cannot find the file specified.

Direct docker info this docker host

$ docker -H 192.168.20.127:2375 info
 Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
 Images: 0
 Server Version: 1.12.0
 Storage Driver: devicemapper
 Pool Name: docker-253:1-101251423-pool
 Pool Blocksize: 65.54 kB
 Base Device Size: 10.74 GB
 Backing Filesystem: xfs
 Data file: /dev/loop0
 Metadata file: /dev/loop1
 Data Space Used: 11.73 MB
 Data Space Total: 107.4 GB
 Data Space Available: 14.84 GB
 Metadata Space Used: 581.6 kB
 Metadata Space Total: 2.147 GB
 Metadata Space Available: 2.147 GB
 Thin Pool Minimum Free Space: 10.74 GB
 Udev Sync Supported: true
 Deferred Removal Enabled: false
 Deferred Deletion Enabled: false
 Deferred Deleted Device Count: 0
 Data loop file: /var/lib/docker/devicemapper/devicemapper/data
 WARNING: Usage of loopback devices is strongly discouraged for production use. Use `--storage-opt dm.thinpooldev` to specify a custom block storage device.
 Metadata loop file: /var/lib/docker/devicemapper/devicemapper/metadata
 Library Version: 1.02.107-RHEL7 (2016-06-09)
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
 Volume: local
 Network: null host bridge overlay
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Security Options: seccomp
 Kernel Version: 3.10.0-123.el7.x86_64
 Operating System: CentOS Linux 7 (Core)
 OSType: linux
 Architecture: x86_64
 CPUs: 1
 Total Memory: 987.2 MiB
 Name: localhost.localdomain
 ID: FMPB:NCHQ:ERTQ:YQMK:WUSA:QA2T:FCQO:TL7L:IHOH:3Z3Z:EXTV:3YMY
 Docker Root Dir: /var/lib/docker
 Debug Mode (client): false
 Debug Mode (server): false
 Registry:  https://index.docker.io/v1/
 WARNING: bridge-nf-call-iptables is disabled
 WARNING: bridge-nf-call-ip6tables is disabled
 Insecure Registries:
 127.0.0.0/8

The problem has begun.
Now you want docker-machine env to look at the variables of the docke.20.127 host and the following prompt appears

$  docker-machine.exe env docker.20.127
 Error checking TLS connection: Error checking and/  or regenerating the certs: There was an error validating certificates for host "192.168.20.127:2375": open C:\Users\JYC103\.docker\machine\machines\  docker.20.127\server.pem: The system cannot find the file specified.
 You can attempt to regenerate them using 'docker-machine regenerate-certs [name]'.
 Be advised that this will trigger a Docker daemon restart which will stop running containers.

Server.pem does not exist, follow its prompts to create one

$  docker-machine.exe regenerate-certs docker.20.127
 Regenerate TLS machine certs?  Warning: this is irreversible. (y/n): y
 Regenerating TLS certificates
 Waiting for SSH to be available  ...
 Too many retries waiting for SSH to be available.  Last error: Maximum number of retries (60) exceeded

However, after waiting for a few minutes, it was not successfully created and debug was opened.

$  docker-machine.exe -D regenerate-certs docker.20.127
 Docker Machine Version:  0.7.0, build a650a40
 Regenerate TLS machine certs?  Warning: this is irreversible. (y/n): y
 Regenerating TLS certificates
 Found binary path at C:\Users\JYC103\bin\docker-machine.exe
 Launching plugin server for driver none
 Plugin server listening at address 127.0.0.1:58959
 () Calling .GetVersion
 Using API Version  1
 () Calling .SetConfigRaw
 () Calling .GetMachineName
 command=configureAuth machine=docker.20.127
 Waiting for SSH to be available  ...
 Getting to WaitForSSH function  ...
 (docker.20.127) Calling .GetSSHHostname
 (docker.20.127) Calling .GetSSHPort
 (docker.20.127) Calling .GetSSHKeyPath
 (docker.20.127) Calling .GetSSHUsername
 Using SSH client type: external
 &{[-F /dev/null -o BatchMode=yes -o PasswordAuthentication=no -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/  null -o LogLevel=quiet -o ConnectionAttempts=3 -o ConnectTimeout=10 -o ControlMaster=no -o ControlPath=none @ -p 0] C:\Program Files\Git\usr\bin\ssh.exe <nil>}
 About to run SSH command:
 exit 0
 SSH cmd err, output: exit status 255: usage: ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
 [-D [bind_address:]port] [-E log_file] [-e escape_char]
 [-F configfile] [-I pkcs11] [-i identity_file]
 [-L address] [-l login_name] [-m mac_spec]
 [-O ctl_cmd] [-o option] [-p port]
 [-Q cipher | cipher-auth | mac | kex | key]
 [-R address] [-S ctl_path] [-W host:port]
 [-w local_tun[:remote_tun]] [user@]hostname [command]
 
 Error getting ssh command 'exit 0' : Something went wrong running an SSH command!
 command : exit 0
 err     : exit status 255
 output  : usage: ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
 [-D [bind_address:]port] [-E log_file] [-e escape_char]
 [-F configfile] [-I pkcs11] [-i identity_file]
 [-L address] [-l login_name] [-m mac_spec]
 [-O ctl_cmd] [-o option] [-p port]
 [-Q cipher | cipher-auth | mac | kex | key]
 [-R address] [-S ctl_path] [-W host:port]
 [-w local_tun[:remote_tun]] [user@]hostname [command]

Report ssh link error.
How to solve this problem and how to generate the server.pem file?