error "attempt to index global 'keymaps' (a nil value)"

Question

I am on Mac with nvim v0.9.0. I have the following „init.lua“ config loaded:

vim.g.mapleader = '<Space>'
local keymap = vim.keymap
keymap.set("i","jk","<ESC>") // line with the error

Can someone tell me, why vim.keymap could be a nil value, and how to fix it?

Thanks in advance