Playground not showing "Execute playground" button

I opened an existing playground file (iOS) on Xcode 11.3.

The panel with the ">" to execute playground does not show anymore ; the panel that opens at bottom is for debug, with a button "=>" for breakpoints.

I cannot find a way to make this "execute playground" reappear.

Note: I have created a new iOS playground, pasted the complete playground here and everything works normally.

In both cases (old and new playground), the panel at bottom is named "Debug area" (when you hover mouse over the leftmost button "v". But in the old playground, there are buttons to show variables and debug.
In addition, the old storyboard fails to run, where the new one, with exact same code, succeeds.

What am I missing here ?
Playground not showing "Execute playground" button
 
 
Q