I had this issue and this is how I solved it on Ubuntu 18.04.1 (in principal it should work with any Linux version):
First make sure you have LibreOffice and Mendeley installed. In my case I installed Mendeley through their
.deb file and LibreOffice through Snap. The following line will search for libreoffice in the installed snaps on your computer and if it finds anything it will show it:
snap list | grep -i libreoffice
This is what you get:
libreoffice 184.108.40.206 80 stable canonical✓ -
Then you need to know where Mendeley is storing the OpenOffice plugin. so let's search for it:
sudo find / -name "openOfficePlugin"
This is what I got:
/opt/ is not readable without
Now you can copy the openOfficePlugin folder to somewhere that is readable by LibreOffice:
cp -r "/opt/mendeleydesktop/share/mendeleydesktop/openOfficePlugin" "~/Downloads"
Now you have to open the "LibreOffice Writer" and "Mendeley" applications.
From the LibreOffice Writer menu, go to "Tools" > "Extension Manager..." (alternatively in version 220.127.116.11 you can use Ctrl+Alt+E to open the Extension Manager)
Now using the "Add" button, go to the
~/Downloads/openOfficePlugin and open the file (in my case it is
Mendeley-1.19.2.oxt but the version might be different in your case).
It takes about a second to add the plugin and then after restarting your LibreOffice, you have the Mendeley toolbar in LibreOffice Writer. If you have your Mendeley open it should automatically connect to it and you can access your library.