I guess you dont have write permission on Extension folder
What you need to do is try to find if you have Extension folder already
Administer -> system setting -> Directories -
look for CiviCRM Extensions Directory
you should find a path in Civicrm 4.7.x ext folder will be created against the below location -
If folder exist then try to navicate to the location in Command prompt or Winscp and provide write permission to the folder - this should fix the issue
refer to the below link - additional reading
Are there recommended directory ownership and permission settings for CiviCRM files?
I guess this helps!!!