I'm sure you changed in the URL instance_name with you instance name, so that I guess you don't have enough privileges to read this table (sys_db_object).

I'm just a user so I don't have rights to anything admin related.

By the way, here's the code for generating that output e.g. in a background script:

function getChildTables(name, level) {  
	var table = new GlideRecord('sys_db_object');  
	if (table.get('name', name)) {  
		gs.print('\t'.repeat(level) + table.label + ' [' + name + ']');  
		var childTables = new GlideRecord('sys_db_object');  
		childTables.addQuery('super_class', table.sys_id);  
		childTables.orderBy('label');  
		childTables.query();  
		while (childTables.next()) {  
			getChildTables(childTables.name, level + 1); 
		}  
	}  
}  

getChildTables('cmdb_ci', 0);

Ok than I recommend to ask an Admin in your company, she/he can also execute the script posted by @mbourla. You will see then also you companies custom tables in CMDB, if there are some.