I tried to install an extension into my newly installed Brackets. For some reason, it failed! Here are my notes to troubleshoot.
The error I get is:  “Installation failed.  Unknown internal error.”

My first step troubleshooting was to open the developer tools in Brackets. It’s under Debug > Show Developer Tools or F12.
I saw these errors of interest:
- WebSocket connection to ‘ws://localhost:40283/’ failed: Unexpected response code: 401
 - [Extension] Error — could not read native directory: /home/skp/.config/Brackets/extensions/user
 - Unknown installation error extensionManager domain is undefined
 
Note: The last one is the one that happens each time I attempt to install.
I attempted to create that directory…
mkdir ~/.config/Brackets/extensions
That made the error go away but didn’t have anything to do with not being able to install extensions.
I think that this is the real issue, and the ticket is still open:
WebSocket connection failed / Undefined Node
I checked to see if the port was open.  Now, my error is:
WebSocket connection to ‘ws://localhost:34273/’ failed: Unexpected response code: 401 
So, I checked for that port:
netstat -ntlp | grep 34273
No results.  I can see all of the ports with:
netstat -ntlp | grep LISTEN
I can see the node process:
$ ps -ef | grep -i node
skp       9789  9764  0 18:21 pts/20   00:00:00 /opt/brackets/Brackets-node /opt/brackets/node-core
I don’t see that it has any ports open:
sudo netstat -ntlp | grep 9977
Update 6/17/2016: Version 1.7 is updated and fixed now.