Can you also double-check that the existing projects aren’t of type “Library” (project.json → designOptions → outputType = “Library”)? As per documentation, Global variables can’t be created in Libraries.
The blank, new project I tried after your first question and the one that allowed me to select the “Global” scope for my variable was a “plain” Windows project.
My existing project, the one I was complaining I cannot assign a “global” scope to my var, is a Windows-legacy project.
If I create a new, empty Windows-legacy project from scratch I can’t assign any “global” scope to vars.
And no; they are not a “Library” type project. All them are of the “Process” kind.
And in the non-legacy ones you cannot rename them, only delete and create them again, which render them almost unuseful. I wonder why this limitation, but oh well.