List - Errors
Many of the list functions call another Flow to process the list (including Filter, Find, For Each, Map, Reduce, Sort, and Unique). If you encounter an error on one of these cards, you may want to look at the Flow History for the Flow that the function calls since the details of the error may be more evident when viewing data in Flow History for that Flow.
A common error with lists is a Type Conversion error since lists of one type don't automatically convert to another type as easily as individual elements do. Lists of heterogeneous types are especially problematic and should be avoided where possible. If you need to convert the items of a list to a new type, use the Map function to convert the list, or do all of your processing inside of Flow called by a For Each function.
As with all errors, reading the error details will often reveal additional information about the error.