<p>My comment is true: beta sites can't be listed in the flag/close dialog as migration options; one has to suggest that destination in an <code>other (needs ♦ moderator attention)</code> flag. Here's <a href="https://softwareengineering.meta.stackexchange.com/a/5780/119139">a quote from Tim Post on Meta Programmers</a> (this was as also quoted partially <a href="https://meta.stackexchange.com/a/178463/246931">here on Meta Stack Exchange</a>):</p>

<blockquote>
  <p>I'm Tim Post, a community manager for Stack Exchange. I'd like to take a few moments to answer this, and explain a bit about migration paths.</p>
  
  <p>Migration paths between sites are something that we're very cautious to establish, we put them in place only when we're certain that the conduit will:</p>
  
  <ul>
  <li>Help ease confusion for users that ask a question on a site that overlaps with another, ensuring a great question is sent where it will receive the best answers without much additional effort on the part of the question author,</li>
  <li>Be largely beneficial to the target site,</li>
  <li>Not become a party to unresolved topic disputes on the originating site, or between the originating and target site...</li>
  </ul>
  
  <p><strong>We don't establish migration paths to a site that is still in its beta period.</strong> We don't want to send content from one site to another unless we're certain that the target site is a permanent fixture. There are exceptions to this; moderators <em>can</em> migrate relatively new questions to sites not in the current migration path, but this ability should be used sparingly. A question should be of much better than average quality, and all hope of it being on topic for the current site should be exhausted prior to going around the established routes.</p>
  
  <p>When [a site] comes out of beta, this is something that we can revisit. However, for now, <strong>it's simply not possible to add a migration path</strong> [to a beta site]. <sup><sub>[Bolding added for emphasis.]</sup></sub></p>
</blockquote>
