DSolve[k'[t] == s k[t]^a - (n + b) k[t], k[t], t]

{{k[t] -> E^(-6 t) C[1]}}

DSolve[{k'[t] == s k[t]^a - (n + b) k[t], k'[t] == 0}, k[t], t]

DSolve: There are fewer dependent variables than equations, so the system is overdetermined.