wendyml To clarify, are you creating your mailing list by going to Contacts --> Create Contact List? And then when sending your distribution, you are not seeing it in the library list in the "To" field of the email?

Yes that’s it

Hmm, I've never had this happen. I'd go to contacts and confirm that you see the list there. Then I'd try using the search box when you are trying to pull the list into the "To" field.
If you have fewer than 200 contacts, you could also try creating the list from the distribution itself (Select Contacts --> New Contact List).
Final thought is that there might just be a delay in processing your list...you could give it 20 minutes or so to see if that helps.
But otherwise, this might be one to submit to support!