TWRP is the recovery program installed on your phone.
Normally you would start it by starting or rebooting the phone with keeping Volume + pressed.
So first, take off the case from the phone and make sure the volume buttons are ok and not somehow stuck …
… then ignore that no OS warning for the moment and reboot anyway. Best case … your OS might just boot normally (can happen). Worst case … your OS does not boot and we know something’s wrong and can proceed from there.