The long and the short of it is there’s no provision in the Constitution. The powers of Congress are a whitelist, and it would be a stretch even for some of the stretchier clauses in there to allow for Federal ballot initiatives.

That said, I wouldn’t support it. I view State-level ballot initiatives as an anti-feature, at least in my native California. Wouldn’t want that crap making it up to the Federal level.

National Initiative is an existing proposal for doing just that which was proposed by Mike Gravel. Tom Steyer included it in his 2020 platform.

https://en.wikipedia.org/wiki/National_initiative