welcome to the notepad++ community, @Marc-Pouliot
the print all plugin has to be installed manually on newer versions of notepad++ (version 7.6.0 and above)
note: print all only works on 32 bit versions of notepad++
first please download the plugin file print.dll from >>> here <<<
(full page: http://www.scout-soft.com/print/)
then create a folder called print inside your notepad++ plugins folder and copy print.dll into this folder and restart notepad++.
side note: the location of your plugin folder will vary, depending on your notepad++ version.
on notepad++ 32 bit version 7.6.3 it will be in %ProgramFiles(x86)%\Notepad++\plugins
so print.dll should go to: C:\ProgramFiles (x86)\Notepad++\plugins\print\print.dll
if you need more help on how to install the print all plugin on your specific machine, please go to the notepad++ menu ? > debug info > copy debug info to clipboardand paste it here, to provide us with your configuration details.