Your code is passing a function as an argument to find. That function takes an element argument (of type Conversation) and returns void (meaning there is no return value). TypeScript describes this as (element: Conversation) => void'
What TypeScript is saying is that the find function doesn't expect to receive a function that takes a Conversation and returns void. It expects a function that takes a Conversations, a number and a Conversation array, and that this function should return a boolean.
So bottom line is that you either need to change your code to pass in the values to find correctly, or else you need to provide an overload to the definition of find in your definition file that accepts a Conversation and returns void.