client[i] = { -1, INVALID_SOCKET };
That is the syntax to initialise a structure but you are trying to set the elements of an existing structure.
You have to use something like
client[i].member1 = -1;
client[i].socket = INVALID_SOCKET;
or
ClientType emptyClient = { -1, INVALID_SOCKET };
client[i] = emptyClient;