I am unable to copy one application between 2 project spaces. The issue is that I am not seeing the destination project space doesn't show in the list of project spaces in the source project space with the same login account.
Is the user a member of both domains? Does the account have admin permissions or "Release Management" permission? I don't think that's necessary to show the domain in the dropdown, but good to check. Are you trying to link the two project spaces, or just copy the app?
Admin permission should be sufficient. I'm not certain why the project space doesn't show up. Couple more questions:
Is this on Dimagi's environment, or a self-hosted instance?
Does your user show up in the list of web users on both project spaces? If you have access as a site-wide superuser or via enterprise permissions, it might not. You need to be a direct member of both domains for this to work.
If it's not one of those things, you might try reaching out to support from within the app so they can debug your specific case.