how can I get value of my custom settings PrefixGroup__c before insert record in Apex class. This is my code but is not working.

Map<String, powerBI_authentication__c> setting = powerBI_authentication__c.getall();
system.debug('++setting' +setting);
powerBI_authentication__c PrefixGroup = setting.get('PrefixGroup__c');    
system.debug('++PrefixGroup' +PrefixGroup);