Pro Sites won't install. It triggers an error

Hello,

I am having some trouble Network activating Pro Sites. Pro Sites won't install. It triggers an error:
Parse error: syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM in /MyWPMSInstallRoot/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Module/Taxamo/lib/Taxamo/Swagger.php on line 185

This is on a new WPMS install with no other plugins or themes active except
iThemes Security.

Do you have any idea what's causing this?

Thank you