Hi Tunahan,
if you are logging in as user SYSTEM (for testing purposes), you should have access to all catalog groups and tiles. This error shouldn't display for user SYSTEM.
For all other users, you need to ensure you have assigned the AppSite and Widget access roles to the user (if you want to see this particular cockpit admin Fiori AppSite). The user will also need the SITE_DESIGNER role in order to make any changes to the layout. Lastly, for this particular Fiori AppSite (administrative) your user will also need the HANA admin roles Administrator and Monitoring.
Maybe try creating a new user and accessing? Perhaps the user your were previously using had their permissions modified?
If you're still stuck, please let me know and I can provide some more information.
Cheers,
Hagen