One possibility is a re-sequencing issue:
You have a set of sheets numbered A, B, C. You want to renumber them, B, C, D.
If you use SetParameterByName you will see the “existing sheet” warning as the numbers are in use when the procedure begins: for example, when A is renumbered B you will see the warnings as B is still in use (sheet B hasn’t been renumbered yet), and so on, hence the problem with resequencing.
If that is the problem use BimorphNodes Sheet.RenumberRename which supports resequencing by circumventing the above limitation.