I am getting below error.
Didn't understand relationship 'Invoice_CU__r' in FROM part of query call. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name.
SOQL:
select Actuals__c,(select id,Total_Price__c from Invoice_CU__r) from Sales_Quota__c
Sales_Quota__c
(master)Invoice_CU__c
(child) has lookup relationship.