How to fix this `Omni completion (^O^N^P) Pattern not found` error?

Question

I am trying to set up syntax keyword based omnicompletion in vim.

I have defined a few syntax keyword Constants in ~/.config/nvim/syntax/spl.vim
They are correctly detected and highlighted.
They are also showing up in :syntax list as below:

:syntax list                                                                                                 
--- Syntax items ---
Operator       xxx match #[=+\-*\/%]#
String         xxx match /".*"/
Comment        xxx match +\/\/.*+
Constant       xxx SWAP_OUT FREE_PAGE_TABLE INT_CLOSE DISK_SIZE GET_CODE_PAGE IDLE_PROCESS INT_LOGOUT
                   CONTEXT_SWITCH MAX_FILE_SIZE INT_DELETE WAIT_TERMINAL INIT_PROCESS MAX_BUFFER
                   RESOURCE_TABLE_OFFSET PAGE_SIZE INODE_TABLE WAIT_DISK FILE 
                   .. truncated ..

:set omnifunc outputs omnifunc=syntaxcomplete#Complete

But still Ctrl+X Ctrl+O shows Omni completion (^O^N^P) Pattern not found

How to fix this?

Vim version is: NVIM v0.4.2

set ft?
  filetype=spl
scriptnames
  ...
  ftdetect/spl.vim
  syntax/spl.vim
  ...