If, on your PHP backend app you’re using Symfony Doctrine and face this error:
error: "[Semantical Error] The annotation "@Nelmio\ApiDocBundle\Annotation\ApiDoc" in method (...) was never imported. Did you maybe forget to add a "use" statement for this annotation? in (...) (which is being imported from "(...)"). Make sure annotations are installed and enabled."
composer update nelmio/api-doc-bundle
If still not working, try the all composer install:
and, in last case: