My raspberry always flashes the lightning icon and there's a warning a saying that. Is it okay to use my Raspberry pi on that condition? Can it damage the raspberry? I don't have an official power supply I currently use my phone charger
Actual hardware damage is very unlikely. The real risk is data loss: you can't trust your system to store data reliably, be it on the SD card, or an external storage such as USB thumb drives.
It's probably not a big problem if you just want to learn a bit of Linux. The moment you try to use the Pi in an actual project, it becomes a problem. You'll likely have strange unexpected failures no one else has (so not solvable by a Google search), which can waste a lot of your time.