You do not want users to execute queries while in the Order_Items block, so you write the following key-Exeqry trigger for the Order_Items block:
GO_BLOCK('orders');
EXECUTE_QUERY;
This works fine except that in the Order_Items block when you click Enter Query first and then click Execute Query, you get the error message “FRM-41009: Function key not allowed.” What can you do to correct this? 

Set the Fire in Enter-Query Mode property of the Key-Exeqry trigger to Yes. 

Add the following as the first of the key-Exqry trigger to Yes.
SYSTEM.mode := 'NORMAL'; 

Delete the Key-Exqry trigger and define the following Key-Entqry trigger for the Order_Items block:
GO_BLOCK ('orders');
ENTER_QUERY; 

Delete the Key-Exqry trigger and define the following Key-Entqry trigger for the Order_Items block:
ENTER_QUERY;
GO_BLOCK ('orders'); 

Change the key-Exqry trigger to:
IF : SYSTEM.MODE := 'ENTER-QUERY' THEN
GO_BLOCK ('orders');
EXECUTE_QUERY;
ELSE
MESSAGE ('you cannot execute a query for the Order_Items block'):
END IF;
Define the following Key-Entqry trigger for the Order_Items block:
GO_BLOCK ('orders');
ENTER_QUERY 

Change the Key-Exeqry trigger to:
IF :SYSTEM.MODE != 'ENTER-QUERY' THEN
GO_BLOCK('orders');
EXECUTE_QUERY;
ELSE
MESSAGE ('You cannot execute a query in this block');
ENTER_QUERY;
GO_BLOCK ('orders'); 

You cannot change this behavior, but you should use the On-Message trigger to display a more meaningful message.