Go to /usr/share/redmine/plugins folder, download, and unzip plugins here.
Go to /usr/share/redmine and run:
bundle install --without development test bundle exec rake redmine:plugins NAME=plugin's name RAILS_ENV=production
Go to the public/plugins_assets folder and check if there are plugins folders or not. if not copy them from plugins folders.
An example is the redmine_contacts plugin. Change the plugin's name if needed.
cd /usr/share/redmine/public/plugins_assets mkdir redmine_contacts cp -r /usr/share/redmine/plugins/redmine_contacts/assets/ /usr/share/redmine/public/plugins_assets/redmine_contacts/assets service apache2 restart
Open your browser and check if Redmine is working without any problems.