Symfony2 BootstrapBundle: There is no "less" filter

Look a bit higher up in the config.yml you likely already have a assetic declaration that came with the installation. You will need to add the new config to that already defined item. (Because your new declaration will be ignored.)

In a standard install you probably have something like

# Assetic Configurationassetic:    debug:          "%kernel.debug%"    use_controller: false    bundles:        [ ]    #java: /usr/bin/java    filters:        cssrewrite: ~        #closure:        #    jar: "%kernel.root_dir%/Resources/java/compiler.jar"        #yui_css:        #    jar: "%kernel.root_dir%/Resources/java/yuicompressor-2.4.7.jar"

Add the new config to that assetic item, making it:

# Assetic Configurationassetic:    debug:          "%kernel.debug%"    use_controller: false    bundles:        [ ]    #java: /usr/bin/java    filters:        cssrewrite: ~        #closure:        #    jar: "%kernel.root_dir%/Resources/java/compiler.jar"        #yui_css:        #    jar: "%kernel.root_dir%/Resources/java/yuicompressor-2.4.7.jar"        less:            node: /usr/local/bin/node            node_paths: [/usr/local/lib/node_modules]            apply_to: "\.less$"        cssrewrite: ~