changed the spark plugs on a 2004 subaru outback 6 cylinder now check engine light comes on

You purchased a great car with a great engine. It will be difficult to determine or diagnose the trouble with out scanning the ECU. Some parts stores will scan your ECU for free. In my area, Advance Auto will hook up and scan for free. You may find this could be as simple as a loose gas cap or may be an accidently displaced sensor connector (given the recent maintenance activities). Best of luck. Hope its an easy code to fix.

Have codes read. Auto zone will do this for free. Then post codes back on here and we will be able to better help you. Also might just take a minute to double check your work. All plug wires fully plugged in? Any wiring harnesses get knocked loose. Ect...

also make sure the sparg plug wires are seated firmly to the spark plugs amd double check the spark plug wires to make sure they are on the right spark plug this would cause a misfie ( I believe) whcih would trigger the check engine light . It is really ha5d to know without the code as stated before I hope this help and wish you the best of luck