So my 1999 Subaru Outback, despite having changed the o2 sensors as well as one other sensor the error code reader said was the issue the check engine light is not on nor does it come on or flash. But when I try to smog it the check engine light comes up in the system with no error codes.....any advice?

When did you clear the codes?  Some of the emissions folks check for the monitors and if you have just cleared the CEL you won't have enough miles on the car to be OK with the monitors.