I created a modpack with this mod for myself and there is no crafting recipe for the sign buttons.
To make sure it's not a problem with other mods I created a modpack with only sign buttons and JEI and it still doesn't work.

jei-1.21.1-neoforge-19.21.0.247
SignButton-1.21-2.6.0
Neoforge 21.1.80