Why do it say error occurred when I try to update my phone

If you see an error when you update or restore your iPhone, iPad, or iPod, click here --> https://support.apple.com/HT201210

"Learn what to do if you see an error code or alert when you use your computer to update or restore your device."

If your iPhone, iPad or iPod touch won’t update - "You might be unable to update your iPhone, iPad, or iPod touch wirelessly — or over the air — for one of these reasons", with more detail in this link. --> https://support.apple.com/HT201435

" Not enough available storage space on your device

Downloading the update takes a long time

Unable to reach the update server

The update doesn't complete"

If you need more space for an update - https://support.apple.com/HT203097

Things to try:

Plug your device into your computer to update using your computer.

Delete content from your device and try to update again.

Manage the storage on your device with recommendations.

If you can't update or restore your iPhone, iPad, or iPod touch - You can put your iOS device into recovery mode, then restore it using Music/iTunes.

https://support.apple.com/HT201263