Create a separate integration flow to handle the error message, and then call it using the Throw New Fault action if the order item count is greater than 10.

Create a scope, add the Throw New Fault action using an If-then branch to check the order item count, then throw the fault if the count is greater than 10.

Implement the validation rule using a For-each action, and then use the Throw New Fault action within the first iteration if the count is greater than 10.

Add the Throw New Fault action as the first activity in the main flow defining the error message, then configuring the Skip condition to be if the count is not greater than 10.