Hi,Try to uninstall or Purge Wkhtmltopdfhttps://www.thelinuxfaq.com/ubuntu/ubuntu-17-04-zesty-zapus/wkhtmltopdf?type=uninstall

Then re-install it 

 You can follow the below command.

sudo wget https://github.com/wkhtmltopdf/wkhtmltopdf/releases/download/0.12.5/wkhtmltox_0.12.5-1.bionic_amd64.deb
sudo dpkg -i wkhtmltox_0.12.5-1.bionic_amd64.deb
sudo apt install -f



And restart your odoo service.

Hope it helps

There is a parameter "bin_path"  in your configuration file. Copy the path to the wkhtmltopdf executable and paste it there. Restart server. 

If you installed with windows all in one installer the wkhtmltopdf exe should be in "thirdparty" folder in the root folder of your installation. Also make sure it is using the right conf file