iphone 6s keeps showing error failed to update to ios 14.8

Note the details about updating to an iOS 14 version in the first document:

Click on a link below for full instructions about how to update with wi-fi or 5G (see: Use 5G with your iPhone

https://support.apple.com/HT211828), or using a computer if you have storage or connectivity issues.

Update your iPhone, iPad, or iPod touch. Click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

I already done iCloud backup and memory space left 3GB but iPhone 6s cannot update iOS14.8.

It possibly can't, depending upon the iOS you're starting from.