Unable to set the Create PDF plugin

Hi

I am trying to setup the Create PDF. First I click the Test Server Requirements and I get back the following: 

PDF Conversion Program
This plugin is using: /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/wkhtmltopdf/wkhtmltopdf-0.12.5-1.ubuntu18.04_bionic_amd64
Plugins settings: CREATEPDF_BINARY_FILE: auto-detected
If all the test below don't pass you can try: download updated binaries and testing all binaries
Warning: shell_exec() has been disabled for security reasons in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF_pluginsMenu.php on line 176
Failed! Run external program on server
Testing shell_exec() with command: pwd
Result: 
Error: No output returned from command!
Warning: exec() has been disabled for security reasons in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF_pluginsMenu.php on line 107 Warning: join(): Invalid arguments passed in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF_pluginsMenu.php on line 108
Failed! Test running PDF conversion program on server
Testing pdf binary with command: /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/wkhtmltopdf/wkhtmltopdf-0.12.5-1.ubuntu18.04_bionic_amd64 -V
Result: 0 bytes returned
Output: 
Error: No output returned from command! (Exit code: -1)
Passed! Create and remove temp file on server
Creating temp file: /var/www/vhosts/vascularregistry.gr/httpdocs/registry/createPDF_856fc8c9adb9c204e8bf45225ee9d504.tmp
Removing temp file: /var/www/vhosts/vascularregistry.gr/httpdocs/registry/createPDF_856fc8c9adb9c204e8bf45225ee9d504.tmp
Passed! Test downloading content from remote URL
Downloading content from url with: getPage('http://www.google.com/');
Result: HTTP status code 302 (view source, html)
Passed! Test downloading content from this server
Downloading content from url with: getPage('http://vascularregistry.gr/');
Resolve hostname 'vascularregistry.gr' to IP address: 185.138.42.30
Result: HTTP status code 301 (view source, html)
Check that this is returning your website's homepage!
Warning: shell_exec() has been disabled for security reasons in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF.php on line 169 Notice: Undefined variable: outputArray in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF.php on line 170 Notice: Undefined variable: returnCode in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF.php on line 170 Warning: exec() has been disabled for security reasons in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF.php on line 170 Notice: Undefined variable: outputArray in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF.php on line 171 Notice: Undefined variable: returnCode in /var/www/vhosts/vascularregistry.gr/httpdocs/registry/plugins/createPDF/createPDF.php on line 175
Failed! Test converting a web page to a PDF
Created PDF with: HTML from: https://www.google.com/
Result: 0 byte PDF returned
Error: Conversion program returned errors:
Temp file didn't get created, check for: 1) wkhtmltopdf errors and 2) make sure server is able to connect to itself internally! 

Then, I try the testing all binaries, but nothing also.

Any suggestions how to proceed?

Regards,

Andreas Lazaris