Make sure you run this command:
composer global require "fxp/composer-asset-plugin:~1.2.0"
Also check your composer.json for the following:
Deprecated but default option from Yii2:
"extra": { "asset-installer-paths": { "npm-asset-library": "vendor/npm", "bower-asset-library": "vendor/bower" } },
New option
"config": { "fxp-asset": { "installer-paths": { "npm-asset-library": "vendor/npm", "bower-asset-library": "vendor/bower" }, } },
Then run composer update if you already have a composer.lock file, or composer install if you don't already have a composer.lock file.
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