I get this error:
The expression 'length(activity('Copy Excel to Cache Sink').output.value)' cannot be evaluated because property 'value' doesn't exist, available properties are 'runStatus, effectiveIntegrationRuntime, billingReference, reportLineageToCatalog'.
I have a DataFlow. It output to activity

In my ForEach Activity in my Pipeline I try to do the following : @activity('Copy Excel to Cache Sink').output.value
BUT the error returns like this