You can find out if your USB accessories are enabled on your iPhone by going into settings, opening Face/Touch ID & Passcode and signing in using your passcode.
If your accessories are not enabled on your phone (many options are grayed out and cannot be changed), then you will need to enable this by setting a passcode. Once there is a passcode, it should let you change your accessories.
The video shows how the iPhone setting changed based on when there is a passcode and when there isn't.
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⦿ For New models of iPhone and iPad,
- please ENABLE/toggle on the USB ACCESSORY settings of your phone because most iPhones, it will require the USB setting to be ENABLED for it to work perfectly as expected. Or even any type of Normal Flash Drives, won't properly work as expected if USB ACCESSORY setting is not enabled or on.
How to ENABLE iPhone USB ACCESSORIES:
1. Go to the Home Screen
2. Then tap SETTINGS (gear icon)
3. Scroll down and look for FACE ID and PASSCODE. It will require you to enter a passcode, the passcode to enter is your Phone Lock Passcode/password)
4. Then scroll down and look for ACCESSORIES only. Then toggle it ON to ENABLE the USB ACCESSORIES.
⦿ For Old models of iPhone and iPad, the one that has the Home Button.
Settings > Touch ID and Passcode > USB Accessories.