Have you tried navigating to Admin > System Settings and raising the "Maximum number of Sugar Dashlets on Homepage" setting? By default, this is set to 15, but you should be able to raise this, assuming you're trying to add a 16th dashlet to your home page. When hitting this limit, you should see an error like this as well: However, this popup looks to be the error being called in the message you posted above.
Hi Alex Sir, I did the same thing on config_override.php,
$sugar_config['max_dashlets_homepage'] = '100';
then did repair then it works, then I did logout and login again the same thing is happened and I am unable to add the dashlets but whenver did the repair it worked.