Why my 14.8.1 software is not downloaded in 6splus iPhone

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

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

https://support.apple.com/en-us/HT206504

What does iCloud back up?

https://support.apple.com/en-us/HT207428

Read what your screenshot says.

You don't have enough storage for the update.