iPhone 8 Plus will not update

Hello ~ You may not have enough device storage:

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

What's the difference between device storage ... - Apple Support

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

~Katana-San~

There may be several reasons, if you can post a full screenshot ,it may help. In the meantime,

Make sure you:

• Are using a WiFi connection (not a hotspot).

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Are not using a VPN connection.

• Do not have power save mode turned on.