Why This Happens
You can’t make a post for the whole school because your account only has access to certain Houses.
Selecting a House tells the system which group of students your post should go to. Without selecting one, the system blocks posting.
There are two causes. Work through them in order:
No House is selected on the Homeboard. Fix this with the steps below.
You are not a member of the House. Being able to filter to a House is not the same as belonging to it — see Still can't post? below.
Step-by-Step
Open the Homeboard.
Choose your House (e.g., Blue House).
Navigate to Activity Feed and click on New Post
Write your message and Publish.
Still can't post? Check House membership
This is the more common cause. To create an Activity Feed post for a House, the staff member must be a member of that House — not merely able to see or filter to it. A staff member can often select a House on the Homeboard and still be blocked from posting to it, because selecting is not the same as belonging.
Having the Create posts permission enabled on their role is not sufficient on its own. Both are needed: the permission, and House membership.
To add the staff member to the House:
Go to Admin Console → Houses.
Edit the house.
Add the staff member as a member and save.
Ask the staff member to refresh Orah, then try creating the post again.
Summary
You must select a House on the Homeboard before creating a post, and you must be a member of that House. This ensures your post goes only to the students and Houses you have permission for.