zenith yewSpiffyHUD needs FancyMenu v3.5.0 or newer to load, so it won’t run with the 1.20.4 version of FancyMenu (which is still on 3.3.2). Because that dependency is hard-coded, there’s no workaround on 1.20.4—until a 3.5+ build is released for that Minecraft version, you’ll have to move to a version of Fabric that ships FancyMenu 3.5+ (currently 1.21.1/1.21.5) to use SpiffyHUD.
You can read more about how Spiffy relies on FancyMenu v3 in the FancyMenu Getting Started guide.