Why I can’t downloaded ios14

For an iPhone 7 Plus, you can update to iOS 15.7.6. To accomplish that, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

The iPhone 7+ can use iOS 15.7.X. When updating iOS, you are required to update to the latest version available for that device.

i need to download ios14 because I need some app on my phone

Go to general and software update if you see there update then download.