Note: see my updated findings below...
I want to initialize a variable with the current day number. So I added an "Initialize variable" action to my Flow, filled in the Name and the Type fields (type is String). However, I cannot get the Value field to accept expressions. The expression I think I need is:
formatdatetime(utcnow(), 'dd')
But when I enter that, the value will be the literal text "formatdatetime(utcnow(), 'dd')". I searched on the internet and found examples of flows that display a separate expression editor. But that isn't displaying for me. Does anybody know how to use expressions in the "Initialize variable" action?

Update: I noticed the expression editor popup does display when editing Flows in a web browser. However, when I'm editing from within my Microsoft Teams client (macOS) it doesn't. After some more testing it appears the expression editor popup only display when the Teams client window is large enough. When the Teams client is displayed full screen on a default Macbook 13 inch. display, the expression editor doesn't appear. The responsive design of the Flow editor clearly fails.
Now, in addition to getting this responsive issue fixed, I think using a separate popup for the expression editor is bad UX. If a field supports expressions, the field itself should offer a way to trigger the expression editor. This could be as simple as adding a link just below the field that opens the expression editor.