The method fullPostPage() does not exist on Cycle\ORM\RepositoryInterface. It seems like you code against a sub-type of Cycle\ORM\RepositoryInterface such as App\Blog\Post\PostRepository.
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation
$output of type Psr\Http\Message\ResponseInterface is incompatible with the type string expected by parameter $string of Psr\Http\Message\StreamInterface::write().
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation