I can see that the validation error is resolved. However I am wondering why that error is not occuring in another process where the .Rows is left out (?)
Always make sure the syntax you’re using is correct. As mentioned by @KarthikByggari it must have been an ARRAY of data rows instead of a DATATABLE which is an OBJECT of System.Data.Datatable class.
An array of data rows can be indexed, not a datatable object. However, a datatable would have Rows() as one of its properties which is a collection of Datarow objects, and hence can be indexed.