Problem
I added an HTML Macro to my page and published the page without adding any code/content to the body of the HTML Macro.
Now I cannot add any code/content to the body of the HTML Macro when I try to edit the page again.
Solution
This is a known issue with the "v1" Atlassian Confluence page editor
This does not happen with the "new" (a.k.a. "fabric") Atlassian Confluence page editor
Delete the empty HTML Macro and add another one in its place. Add code/content to the body of the macro before you publish the page using the v1 page editor.