Addresses must be revealed first by sending a transaction to it. Sending a transaction to it will save the related public key (the « manager » in the error message) in the context. It is implemented in this way because saving in the context and keep data in the context has a cost. You can find more information here.
No, generated addresses do not need to be activated (except fundraiser addresses). However, the first operation shell your sign and broadcast must include a reveal operation. This operation reveals the public key associated to a public key hash, so others can verify the signatures associated to the public key hash.