Auto lock greyed out on iPhone

Do you have Low Power Mode enabled? Look at the battery indicator. If it's yellow, you have Low Power mode enabled and it sets the time out to 30 seconds.

Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support

Try a forced restart.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

Force restart iPhone - Apple Support (IN)

If not a managed device, is it possible you have an Exchange mail account from your work? Either of those could result in your autolock being set to the low setting.

Is your iPhone a managed device?