So the Mechanics Source actually has to be a 'Source' article. Most of you will not have seen this in action yet but the link below might be able to assist. It's a 5e Game System Export that makes use of Mechanics and has a Source selected.
D&D 5e - Unearthed Arcana: Traps Revised
Think of your source as the home page for the content. This will be made much clearer once you see the free content. It's the first page the user will see once they import the content.
The error message you have got has been experienced by others since the last patch. I suspect LWD are already working on this. I recommend
logging a bug though so they can get as much information on this as possible.
Never be afraid to ask for help. I strongly suggest people
join this facebook group. There's 173 RW users currently asking and answering questions daily. It's not official support, but it's a helpful community.
If you have not seen them yet I would also suggest watching my videos on this topic:
Realm Works - How to Export a Realm (Full or Partial Export)
Realm Works - Importing a Shared Realm
Realm Works - How to Copy a Realm