Hi @Mahmoud_Ghoneem,
have you checked
- firewall blocking these intra-zone calls
- if you have libssl1.0-dev missing (normally missing in Debian 9)
Otherwise, try to grab the html for your sales invoice (refer to Wkhtmltopdf layout error - #4 by lasalesi) and manually run it through wkhtmltopdf to see detailed errors.
Hope this helps.