I replace my battery and alternator on my 2015 chevy sub and my battery light still come on and my volts read 9 on my dash. does anyone know what is worry still??

Did you replace at exactly the same time? No days and miles between replacements

Have you used a digital multimeter to try to verify anything, Battery and charging system? You can use digital multimeter across battery posts, everything off, what is the reading? If the battery is down, get it charged up. Make sure battery connections are all good. Any applicable trouble codes? You might be able to use a test light for some testing, the bat terminal at generator should be hot all the time, even with key off, is it hot? If it isn't hot, could be fuse problem or wiring circuit. Your charging system is computer controlled. With engine running, use digital multimeter at generator bat terminal, is the reading the same as voltmeter on the dash. Turning on your head lamps may put it in full charge mode, check the reading.

If battery was below 11 volts when you installed new alternator, the new alternator has to work too hard charging battery possibly ruining new alternator. Had it happen many times to my DIY clients. I always put battery on charge during alternator change. If too low you might have killed a cell in battery rendering it no good. Do as Mr oil stated and test with multimeter to see running voltage and load test. Many good videos online to test charging system.