Not able to define mappings without Plug package manager

[fithurriague]

Package manager: Packer.nvim Intention: Set keymappings to jump foward and backward Problem: Mappings depend on <Plug> prefix

imap <expr> <Tab>   vsnip#jumpable(1)   ? '<Plug>(vsnip-jump-next)'      : '<Tab>'
smap <expr> <Tab>   vsnip#jumpable(1)   ? '<Plug>(vsnip-jump-next)'      : '<Tab>'
imap <expr> <S-Tab> vsnip#jumpable(-1)  ? '<Plug>(vsnip-jump-prev)'      : '<S-Tab>'
smap <expr> <S-Tab> vsnip#jumpable(-1)  ? '<Plug>(vsnip-jump-prev)'      : '<S-Tab>'

Need help, really like the plugin over others written in lua or python, I don't really want to migrate to Ultisnipts even though it's a great plugin.

[initial1ze]

<Plug> here does not mean it's vim-plug plugin manager, here is a screenshot from :h Plug.

Mappings provided in the README should work fine. If they are still not working for you, you can try and change them and see if that works.