Robert Charles van Messel apparently worked as a nurse at the Joodsche Invalide, a large care facility for elderly and disabled individuals in Amsterdam. One of his co-workers was Louise van IJssel, whose webpage on Joods Monument provides much of this information about Robert's life and death.
He and Louise were arrested on 16 August 1943 and sent to Westerbork on 26 August 1943. They were deported to Auschwitz on 31 August. Robert died there on 31 March 1944.