Update 15.2.1
My phone won't update. It says an error has occurred.
iPhone 11 Pro Max
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Optimize the range of the mobile network, see this Apple article: If you see No Service or Searching on your iPhone or iPad
https://support.apple.com/en-us/HT201415
Check also the Wi-Fi network, see this Apple article: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
https://support.apple.com/en-us/HT204051
You need about 6 GB device storage, see this Apple articles below:
If you need more space for an update on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT203097
How to check the storage on your iPhone, iPad and iPod touch
https://support.apple.com/en-gb/HT201656
Clear Other storage on your iPhone or iPad
https://support.apple.com/en-us/HT211686
What's the difference between device storage and iCloud storage?
https://support.apple.com/en-us/HT206504