Can't resolve multiple constant drivers for net "next_state"

I think the reason for the error message is that there are two events which can affect the value of nextstate which are not mutually exclusive. Consider what would happen if there was a positive edge on clock at the same time that reset changed. What would nextstate become?

I'd use a synchronous reset, i.e.

always @ (posedge clock)

if(reset)

begin

// Reset nextstate

end

else

begin

// State machine case statement

end

If you re-order your code in this way you might find it works.