The combination jk is not thought as a general replacement for <esc> in all modes. It's just for insert mode to save you from moving your finger up to the <esc> key. In all other modes you have to use "normal" <esc>, but then jk just moves the cursor down and up -- so nothing happens.
There are users out there who redefine their keyboard ...
The key syntax with brackets, such as <Esc>, is specific to Vim.
In plain vi, you need to enter an actual ESC character in your mapping.
You can do so by pressing Ctrl+V, followed by ESC, which will insert an actual ESC and will be displayed as ^[.
The command will look like:
:inoremap jj ^[
But note that the ^[ is a single ESC character and is ...