I have same issue today with .NET 8 in revit 2025 today, and this solution resolved issue to me :

1. Install Epplus package (I'm using 7.6.1 in .NET 8)

  <PackageReference Include="EPPlus" Version="7.6.1" />

2. Add Resolve Assembly in to App.cs when your add-in startup

 public Result OnStartup(UIControlledApplication application)
    {
AppDomain.CurrentDomain.AssemblyResolve += CurrentDomainOnAssemblyResolve;

}
private Assembly? CurrentDomainOnAssemblyResolve(object sender, ResolveEventArgs args)
    {
        // Get assembly name
        var assemblyName = new AssemblyName(args.Name).Name + ".dll";

        // Get resource name
        var resourceName = Assembly.GetExecutingAssembly().GetManifestResourceNames().Where(x => x.EndsWith(".dll"))
            .ToArray().FirstOrDefault(x => x.EndsWith(assemblyName));
        if (resourceName == null)
        {
            return null;
        }

        // Load assembly from resource
        using (var stream = Assembly.GetExecutingAssembly().GetManifestResourceStream(resourceName))
        {
            var bytes = new byte[stream!.Length];
            stream.Read(bytes, 0, bytes.Length);
            return Assembly.Load(bytes);
        }
    }

3. Rebuild your project again and open revit, problem should resolve.