Hi!
The message is that on your page only the order is available. If you want to use the entity 'Klant’, you have to retrieve this object over association and not as input parameter.
Input parameter is: Order
First activity: Retrieve over association: Klant
Then you can use that expression.
Hope this helps!
In your expression you only have to type: $Klant/Kredietlimiet < $Order/Totaalprijs
The determine the outcome of the statement using the connectors (sequence flows)