The difference between a chatbot and an RPA engine is similar to a difference between a mouse and a mammoth. By the names,
ChatBot – A bot programmed to chat with a user (human) like a human. Possibly to recommend a pre-learned resolution to a particular problem that user faces or to take a request and instruct backend systems to provision a request.
RPA (Robotic Process Automation) – A bot programmed to automate a manual business process of executing a task or an activity within a business function. A business function can be like HR/Finance/Procurement etc. A chatbot can be a medium to feed an input into an RPA engine but cannot replicate the features of an RPA engine.