Error : We can’t find the 'Child Campaign' custom button on the Campaign object.

I'm working on the "Create a button to launch a flow"

Trailhead challenge. 

I’ve created a

• Label: Child Campaign
• Name: Child_Campaign
• Behavior: Display in existing window without sidebar or header
• URL:

/flow/Create_Child_Campaign?campID={!Campaign.Id}&campName={!Campaign.Name}&campOwner={!Campaign.OwnerId}&campStartDate={!TEXT(Campaign.StartDate)}&retURL=/{!Campaign.Id}

I confirm:

• The button is created with the right config and URL
• It’s added to the Campaign Page Layout (under Custom Buttons)
• It’s also added to the Lightning Record Page via Dynamic Actions

But Trailhead still shows this error:

What could I be missing?

Let me know if you want a version to cross-post on Salesforce Stack Exchange too.