If you're mapping a custom value from an object you've created in a previous step, it may not exist yet. You can resolve this by adding a delay step after the step where you created the record in order to give the app some time to process.
ExampleIf you have an action that creates a new lead in your CRM, then adds that lead to a new deal, your CRM may not have finished creating the lead before the Zap tries to add the lead to your deal. You can add a delay step after creating the lead to give the CRM time to finish creating the lead first.