I can't comment out multi line in sublime 3. Looked at this Keyboard shortcut to comment lines in Sublime Text 3 already but it is not fixing it Sublime version is 3.2.1 build 3207 . on windows 10. Please help me out if have any tips.
The keyboard binding for commenting is indeed Ctrl+/ (Cmd+/ on MacOS) so in the general case what you're trying should work just fine.
That said, the keys in question are bound to the toggle_comment command and that command requires language specific metadata to know what comments are supposed to look like. For example comments look and work differently in HTML than they do in C or JavaScript. Some file types (such as plain text files) aren't code and thus don't have a concept of comments at all.
If the metadata file for a file type is missing, then the toggle_comment command won't do anything because comments are assumed to not be valid, which seems to be what's happening to here.
The first thing to check would be to examine the bottom right of the window to see if Sublime agrees with you about what the file type is supposed to be. For example, newly created tabs are Plain Text files until you save the file for the first time, and comments are not allowed in Plain Text files.
If the file type doesn't say what you think it should, you can click it to open a menu and select the appropriate type for that file.
If the type of the file looks to be correct and the command still doesn't work, then the metadata file needed is not being provided in the package that is adding support for that language. In that case you should raise an issue with whoever is responsible for the package in question and get them to add the appropriate file.
Based on the comments on your question, you think you're editing JavaScript files but Sublime thinks you're editing an ActionScript file instead. The JavaScript package includes the appropriate file, but the ActionScript package does not.
So in your particular case, your best bet is to switch the file type to JavaScript and your problem should go away. If you actually want to be editing ActionScript instead, then you need to create an issue on the Default package tracker and ask for this to be added.