You need to be signed into the Creative Cloud to be able to share to the Creative Cloud. We're currently free in beta, so even if you haven't paid for CC, you should still be able to share right now. One possibility is that you might have been signed out somehow. You should launch your Creative Cloud desktop application and ensure you're logged in, then launch XD again and see if that fixes it for you.
Thanks elainecc, I tried your suggestion but it still didn't work. I am sure my Creative Cloud desktop app is signed in.
It keeps saying "Unable to sync <file> due to server error." I have tried several times on different devices, all got "server error".
Today I found out that preview on mobile device on Adobe XD windows app is supported. So I don't have to sync files to the Cloud.
Thanks for letting us know! It does look like this is an issue with the Creative Cloud desktop application, so you should definitely let them know in that forum.