Hello Aryals,
We understand that the "Select All" option is missing when you're trying to delete your emails on your iPhone.
We've provided some steps below that may be able to assist you with this issue:
• Restart your iPhone
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
• Close an app on your iPhone or iPod touch
iPhone X and later
1. From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
2. Swipe right or left to find the app that you want to close.
3. Swipe up on the app's preview to close the app.
• Update your iPhone, iPad, or iPod touch (Be sure to secure your data with a backup)
Let us know if this helps. Be safe and well!