On $ grunt build I am getting: Warning: Running "sass:server" (sass) task

[m-ret]

This is the grunt config for that part:

    // Compile SASS to CSS
    sass: {
        server: {
            files: {
                '.tmp/assets/styles/main.css': '<%= pkg.app %>/assets/styles/main.{scss,sass}'
            }
        }
    },

Node: v5.12.0 Npm: 5.0.4

Console output:

crsjopgp4581:promo-ways-to-invest berretan$ grunt build
Running "clean:dist" (clean) task
Cleaning .tmp...OK

Running "exec:createDistDir" (exec) task

Running "concurrent:dist" (concurrent) task
>> Warning: There are more tasks than your concurrency limit. After this limit
>> is reached no further tasks will be run until the current tasks are
>> completed. You can adjust the limit in the concurrent task options
    
    Running "copy:css" (copy) task
    Copied 3 files
    
    Done, without errors.
    
    
    Execution Time (2017-06-29 18:22:04 UTC)
    loading tasks  2.1s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 99%
    copy:css       26ms  ▇ 1%
    Total 2.1s
        Warning: Running "sass:server" (sass) task
    >> Error: ".navbar-form" failed to @extend ".form-inline".
    >>        The selector ".form-inline" was not found.
    >>        Use "@extend .form-inline !optional" if the extend should be able to fail.
    >>         on line 313 of app/bower_components/bootstrap-sass-official/assets/stylesheets/bootstrap/_navbar.scss
    >> >>   @extend .form-inline;
    >>    ----------^
    Warning:  Use --force to continue.
    
    Aborted due to warnings.
    
    
    Execution Time (2017-06-29 18:22:04 UTC)
    loading tasks   2.1s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 87%
    sass:server    314ms  ▇▇▇▇▇▇ 13%
    Total 2.5s Use --force to continue.
        
        Aborted due to warnings.
    

Execution Time (2017-06-29 18:22:02 UTC)
loading tasks    1.1s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 26%
concurrent:dist  3.2s  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 73%
Total 4.3s

crsjopgp4581:promo-ways-to-invest berretan$ node -v
v5.12.0
crsjopgp4581:promo-ways-to-invest berretan$ npm -v
5.0.4
crsjopgp4581:promo-ways-to-invest berretan$ 

I am using nvm.

Any suggestions?