There is at least one abstract method in this class. Maybe declare it as abstract, or implement the remaining methods: hasProperty, p, properties, property
The expression $this->externalUrl; of type Charcoal\Cms\Section\Translation|string|null adds the type Charcoal\Cms\Section\Translation to the return on line 36 which is incompatible with the return type declared by the interface Charcoal\Cms\SectionInterface::externalUrl of type string.