Why is my check engine light still on after I changed the back 1 sensor 1 o2 sensor?

Code P2251 references an issue with the oxygen sensor or sensor circuit for the Bank 1 Sensor 1 oxygen sensor.
Have you had any repairs done for this fault code?
Looking through the diagnostic path the fault could be the oxygen sensor itself, the wiring to and from the computer to oxygen sensor or lastly and the least likely is the computer itself has failed.
Clearing the fault codes won't make it go away. When you clear the code this will make the computer restart it's self tests and if the computer see something wrong with this oxygen sensor again it will cause the check engine light to come on again. Also if you don't have all the emission monitors passed it won't pass emission inspections either. You'll need to address this P2251 fault code. Most likely it's the sensor itself but having it check will rule out other possibilities.