vagrant up 报错请帮忙

$ sudo vagrant up
ringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'centos7' could not be found. Attempting to find and install...
default: Box Provider: virtualbox
# -*- mode: ruby -*-
# vi: set ft=ruby :

# Vagrantfile API/syntax version. Don't touch unless you know what you're doing!
VAGRANTFILE_API_VERSION = "2"

Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
# All Vagrant configuration is done here. The most common configuration
# options are documented and commented below. For a complete reference,
# please see the online documentation at vagrantup.com.

# Every Vagrant virtual environment requires a box to build off of.
config.vm.box = "centos7"

# Disable automatic box update checking. If you disable this, then
# boxes will only be checked for updates when the user runs
# `vagrant box outdated`. This is not recommended.
config.vm.box_check_update = false

# Create a forwarded port mapping which allows access to a specific port
# within the machine from a port on the host machine. In the example below,
# accessing "localhost:8080" will access port 80 on the guest machine.
# config.vm.network "forwarded_port", guest: 80, host: 8080




==========================================



$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
==> default: Box 'centos7' could not be found. Attempting to find and install...
default: Box Provider: virtualbox
default: Box Version: >= 0
==> default: Adding box 'centos7' (v0) for provider: virtualbox
default: Downloading: centos7
An error occurred while downloading the remote file. The error
message, if any, is reproduced below. Please fix this error and try
again.

Couldn't open file /Users/a/vagrant/centos7/centos7
OverlordsMac:centos7 a$