Originally Posted by Ferus

1 - Install Site Kit with an incognito window
2 - Temporarily disable any potential security/ad blocker or firewall software:
3 - Switch your active WordPress theme to the DEFAULT theme
4 - Deactivate all other plugins plugins (temp)
5 - Ensure that your environment fulfill all minimum requirements. Site Kit requires a minimum PHP version or 5.4 and WordPress version 4.7 or later.
6 - If it dont work, repeate steps using a different computer to set up Site Kit