Hi,

 please check the version of wkhtmltopdf installed on your system by using the following terminal command:wkhtmltopdf --version

if not ,install wkhtmltopdf using these commands 

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

Check these references too

forum

Regards