please make sure that correct version of wkhtmltopdf you are using.

please try following steps in order for(ubutnu):

sudo apt-get remove - auto-remove wkhtmltopdf
sudo apt-get purge - auto-remove wkhtmltopdfsudo wget https://builds.wkhtmltopdf.org/0.12.1.3/wkhtmltox_0.12.1.3-1~bionic_amd64.deb
sudo dpkg -i wkhtmltox_0.12.1.3-1 ~ bionic_amd64.deb
sudo apt-get install -f
sudo ln -s / usr / local / bin / wkhtmltopdf / usr / bin
sudo ln -s / usr / local / bin / wkhtmltoimage / usr / bin