Hey!
I play on 1.12 with Forge 14.21.0.2331 and the 3.0.2 version of this mod (I tried the 3.0.0 too), but I can`t craft the backpack. When I put all the ingridients in to the craft table nothing happens.
When I exit the world I can see that the mod is loaded and I can configure it.
Other mods I use: Optifine, Ruins, XaerosMinimap and Worldmap.
If you need any other information I will be happy to provide it!

PS: I have to use this version of Forge, because its the only one Optifine supports right now for 1.12

This specific version of Forge might not have working recipes. Even if there was a way to fix this on Wearable Backpacks' end, it would be a little silly to support an outdated, unsupported and unfinished beta version of Forge for 1.12. Apologies.

For the record, here is the changelog for Forge 2344, the version WB 3.0.2 was built for. As you can see between 2331 and 2344 there were a bunch of fixes and changes regarding the recipe system.