Hi,

You can try to install the ttf-mscorefonts-installer package and clear your font cache ,

sudo apt install ttf-mscorefonts-installer 

sudo fc-cache -f -v

If this works its good and if not, try installing  Wkhtmltopdf package again, and make sure the version is patched with qt

1. First check for the version of wkhtmltopdf 

sudo wkhtmltopdf --version
2. And then install the new version if your having an old one. According to your system config download the package

 https://wkhtmltopdf.org/downloads.html

3. Install it and run the server after clearing your browser cache.

 Hope it helps,

Thanks