Can’t change auto lock setting from 2 minutes, option greyed out

iPad auto-lock will be fixed at two minutes if you have enabled Low Power mode:

Settings > Battery > Low Power Mode

If the problem persists after disabling Low Power Mode, perform a forced-restart of your iPad:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

You might also check that Date & Time and TimeZone are also correctly set for your iPad - as these settings are known to cause various functional issues if incorrectly configured:

Settings > General > Date & Time

If you have a corporate email account set up on the device, the administrators can enforce policies on the device including passcode and lock timeout requirements.

If that’s the case, the only way to change it will be to remove the account from the device. If you re-add the account, the policies will be enforced again.