I opened the frunk manually and then closed it. The warning did not return after doing so.
I started the car several times before opening the frunk & the warning kept appearing.
The now obvious response was to just open & close the frunk.
😃
I pushed the button in the frunk, out of curiosity, now I get the "Frunk system fault. Service required“ message on the dashboard. How do I get rid of it? Do I have to go into a Ford dealer?