The method requiresAtLeastOneElement() does not exist on Symfony\Component\Config...\Builder\NodeDefinition. It seems like you code against a sub-type of Symfony\Component\Config...\Builder\NodeDefinition such as Symfony\Component\Config...der\ArrayNodeDefinition.
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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-call annotation
The expression return $node returns the type Symfony\Component\Config...\Builder\NodeDefinition which includes types incompatible with the type-hinted return Symfony\Component\Config...der\ArrayNodeDefinition.