With Revit’s switch to .NET 8 there is no longer access to the global assembly cache. As a result clr.AddReferenceByName('Microsoft.Office.Interop.Excel, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c') is going to fail.
Lots of good discussions on this on the forum already - @c.poupin has some exceptional posts as well. This post in particular has some good stuff in it: Something wrong when using Microsoft.Office.Interop.Excel in Revit 2025, Dynamo 3.0.2 - #5 by c.poupin
All of this means you will have to update your interop method for the new environment. Sadly Microsoft hasn’t made it easy on developers.
If you do use a Dynamo provided method, I recommend utilizing the OpenXML version as the other one will be deprecated in a coming release.