Why doesn’t my alarm go off when silent mode is on ?

Set the alarm's volume

Go to Settings > Sounds & Haptics. Under Ringers And Alerts, drag the slider left or right to set the volume. As you drag, an alert will play, so you can hear how the volume changes. Turn on Change with Buttons to use the volume buttons on your device to change the alarm volume. If the alarm volume is too low or quiet, learn what to do.