Error installing with composer

[maxxer]

After issuing the composer command:

[...]
  - Installing yiisoft/yii2-gii (dev-master 433701f)
    Loading from cache

Writing lock file
Generating autoload files
Setting writable: backend/runtime ...done
Setting writable: backend/web/assets ...The directory was not found: /home/maxxer/work/gmg/practical-a/backend/web/assets