Oh, I know that the reason for the error is the referenced method is now deleted. I thought that I need something in my Registry handler for the client (excuse me if what I just said makes no sense, I was struggling to find words for what I meant.).
So I deleted the line from my registry handler, and changed
ModelLoader.setCustomModelResourceLocation(johncena, 0, (ModelResourceLocation) johncena.getRegistryName());
to
ModelLoader.setCustomModelResourceLocation(johncena, 0, new ModelResourceLocation(johncena.getRegistryName(), "inventory"));
I also renamed en_US.lang to en_us.lang.
Worked perfectly, thanks!