Vimtex: how to display error messages?

The best way is to use \le, and here is a full list of command of vimtex (from the official document)

   LHS              RHS                                          MODE~
   <localleader>li  |<plug>(vimtex-info)|                           `n`
   <localleader>lI  |<plug>(vimtex-info-full)|                      `n`
   <localleader>lt  |<plug>(vimtex-toc-open)|                       `n`
   <localleader>lT  |<plug>(vimtex-toc-toggle)|                     `n`
   <localleader>lq  |<plug>(vimtex-log)|                            `n`
   <localleader>lv  |<plug>(vimtex-view)|                           `n`
   <localleader>lr  |<plug>(vimtex-reverse-search)|                 `n`
   <localleader>ll  |<plug>(vimtex-compile)|                        `n`
   <localleader>lL  |<plug>(vimtex-compile-selected)|               `nx`
   <localleader>lk  |<plug>(vimtex-stop)|                           `n`
   <localleader>lK  |<plug>(vimtex-stop-all)|                       `n`
   <localleader>le  |<plug>(vimtex-errors)|                         `n`
   <localleader>lo  |<plug>(vimtex-compile-output)|                 `n`
   <localleader>lg  |<plug>(vimtex-status)|                         `n`
   <localleader>lG  |<plug>(vimtex-status-all)|                     `n`
   <localleader>lc  |<plug>(vimtex-clean)|                          `n`
   <localleader>lC  |<plug>(vimtex-clean-full)|                     `n`
   <localleader>lm  |<plug>(vimtex-imaps-list)|                     `n`
   <localleader>lx  |<plug>(vimtex-reload)|                         `n`
   <localleader>lX  |<plug>(vimtex-reload-state)|                   `n`
   <localleader>ls  |<plug>(vimtex-toggle-main)|                    `n`
   dse              |<plug>(vimtex-env-delete)|                     `n`
   dsc              |<plug>(vimtex-cmd-delete)|                     `n`
   ds$              |<plug>(vimtex-env-delete-math)|                `n`
   dsd              |<plug>(vimtex-delim-delete)|                   `n`
   cse              |<plug>(vimtex-env-change)|                     `n`
   csc              |<plug>(vimtex-cmd-change)|                     `n`
   cs$              |<plug>(vimtex-env-change-math)|                `n`
   csd              |<plug>(vimtex-delim-change-math)|              `n`
   tsc              |<plug>(vimtex-cmd-toggle-star)|                `n`
   tse              |<plug>(vimtex-env-toggle-star)|                `n`
   tsd              |<plug>(vimtex-delim-toggle-modifier)|          `nx`
   tsD              |<plug>(vimtex-delim-toggle-modifier-reverse)|  `nx`
   <F7>             |<plug>(vimtex-cmd-create)|                     `nxi`
   ]]               |<plug>(vimtex-delim-close)|                    `i`
   ac               |<plug>(vimtex-ac)|                             `xo`
   ic               |<plug>(vimtex-ic)|                             `xo`
   ad               |<plug>(vimtex-ad)|                             `xo`
   id               |<plug>(vimtex-id)|                             `xo`
   ae               |<plug>(vimtex-ae)|                             `xo`
   ie               |<plug>(vimtex-ie)|                             `xo`
   a$               |<plug>(vimtex-a$)|                             `xo`
   i$               |<plug>(vimtex-i$)|                             `xo`
   aP               |<plug>(vimtex-aP)|                             `xo`
   iP               |<plug>(vimtex-iP)|                             `xo`
   %                |<plug>(vimtex-%)|                              `nxo`
   ]]               |<plug>(vimtex-]])|                             `nxo`
   ][               |<plug>(vimtex-][)|                             `nxo`
   []               |<plug>(vimtex-[])|                             `nxo`
   [[               |<plug>(vimtex-[[)|                             `nxo`
   ]m               |<plug>(vimtex-]m)|                             `nxo`
   ]M               |<plug>(vimtex-]M)|                             `nxo`
   [m               |<plug>(vimtex-[m)|                             `nxo`
   [M               |<plug>(vimtex-[M)|                             `nxo`
   ]/               |<plug>(vimtex-]/|                              `nxo`
   ]*               |<plug>(vimtex-]star|                           `nxo`
   [/               |<plug>(vimtex-[/|                              `nxo`
   [*               |<plug>(vimtex-[star|                           `nxo`
   K                |<plug>(vimtex-doc-package)|                    `n`

And if you forgot them when you're using vim, you can actually type :nn when editing a tex file to see the same thing The result of typing :nn

The vimtex plugin populates the quick fix view with the errors. It should actually open automatically (by default) in a bottom split-window. It is weird that it does not for you. Invoking :copen directly should open the window. Also [q and ]q should let you iterate between errors (but this does not work very well for latex as it is hard to filter spurious and non-spurious messages in the log; LaTeX's log is a mess, in comparison to most compilers).