In the circuit, the CNOT gate is applied in 11 state and it should transform into 10 state. But why is the probability of getting 01 state 100 percent?
You are right, that $CNOT |11\rangle = |10\rangle $ in the usual setting. But the reason why you see the answer as $|01\rangle$ instead is because Qiskit uses little endian convention.
See this documentation to clear thing up.