Can't insert frame break at end of flow

I want a different rastral size for each flow, and I see from previous answers on the forum that I should insert a frame or system break and set its space size property. But I can’t insert a break - if I select the final bar line of the flow, or the first of the next flow, and click “Create Frame Break” in the engrave panel, nothing happens. How do I insert the break (or get different rastral sizes per flow without inserting it?)

Dorico 4, Windows 10.

Select the first note/rest of the new Flow, then create the break.

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