Why can’t I update my iPhone?

Here are the conditions that must be met to update iOS over the air:

• You must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• The phone can not be in Low Power mode
• The battery state of charge must be above 50%
• You must have at least 3 GB of free storage
• You cannot be connected through VPN
• If you had the iOS 13 beta on your phone you must delete the beta profile and restart your phone
• If you are not planning for it to update automatically you should turn this option off, as it will only update when the phone is plugged in, connected to Wi-Fi and locked
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Here’s more info from Apple—>If your iPhone, iPad or iPod touch won’t update - Apple Support

I have no idea.. what happens when you try?

Update your iPhone, iPad, or iPod touch - Apple Support

it’s say update requsted instead of download and install .