encountered error 75 during RESTORE - what next ?

Windows, 

1.

Try to free up iPhone device storage see this Apple articles below:

If you need more space for an update on your iPhone, iPad, or iPod touch

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

How to check the storage on your iPhone, iPad and iPod touch

How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

-> What's the difference between device storage and iCloud storage?

What's the difference between device storage and iCloud storage? - Apple Support

2.

Turn your iPhone on and unlock it -> enter the passcode.

Go to Settings -> set Auto-Lock to Never.

Connect your device to your computer using an Apple USB cable & try also another USB cable.

Make sure your Windows PC is up to date & have the latest version of iTunes.

See also -> Check your third-party security software.

Check the Wi-Fi connection.

If they are On, turn Off Hotspot & VPN.

Disable all AV & Firewalls temporarily while you restore your iPhone.