I cannot download iOS 14 on my iPhone 6 Plus

An iPhone 6 Plus isn't capable of running 13 or above. If you encounter an issue updating it to 12.5.4, describe it in this thread.

(204145)

Danshepardson wrote:

I cannot download iOS 14 on my iPhone 6 Plus

True statement. The iPhone 6 series 7 year old hardware cannot run an iOS version higher than 12.5.4.