Hi there,
I see that “Orphan page (has no incoming internal links)” message you are referring to is about how not having internal links to a page makes it more difficult for search engines to crawl and index pages: https://linkwhisper.com/orphan-page/
And it looks like you are talking specifically about your MailPoet “Manage Subscriptions” page.
In general, most site owners don’t want those pages (manage subscription, unsubscribe, etc) appearing in search results and other places, since they are just for allowing individual subscribers to manage their subscription.
That said, if you would like to, you could create a Custom page on your site, and include internal links to it (from your site header or somewhere): https://kb.mailpoet.com/article/222-customize-your-manage-subscription-page
