I'm having an existing event system solution where I'm using Tridion.ContentManager dll. And finally creating merged dll for event system using ILMerge. Its working fine if I'm using Web 8.5 dlls but i'm not able to create merged dll when using sites 9.1 Tridion.ContentManager dll with version "9.1.0.0". 
Here is the error we are getting :

> An exception occurred during merging: Unresolved assembly reference
> not allowed: Tridion.ContentManager.Data.    at
> System.Compiler.Ir2md.GetAssemblyRefIndex(AssemblyNode assembly)    at
> System.Compiler.Ir2md.GetTypeRefIndex(TypeNode type)    at
> System.Compiler.Ir2md.VisitReferencedType(TypeNode type)    at
> System.Compiler.Ir2md.VisitClass(Class Class)