Unwritable Extensions Directory
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 - 

.../sites/default/files/civicrm/ext

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?

https://forum.civicrm.org/index.php?topic=10545.0

I guess this helps!!!