Help With Business Unit Security

Hello,

Please see attached image.

I have a parent BU and a child BU. The parent should have full access to everything and the child only have access to it's own records.

When logged in as a child BU user, I am unable to create a new record in my custom entities above - despite having organisation level create permissions. Why would that be? I am also unable to read any records for this entity...

The message in the ErrorDetails.txt is "systemform With Id = 422788e6-8c74-40de-975e-e76fa552db9d Does Not Exist"

I haven't enabled security roles on the Main form for any entities so I'm puzzled as to what's going on?