Unable install iOS 15 in my iPhone 7

Does it provide as hint as to why? One you can share with us?

If you see an error when you update or restore your iPhone, iPad, or iPod, click here --> https://support.apple.com/HT201210

If your iPhone, iPad or iPod touch won’t update - "You might be unable to update your iPhone, iPad, or iPod touch wirelessly — or over the air — for one of these reasons", with more detail in this link. --> https://support.apple.com/HT201435

If you need more space for an update - https://support.apple.com/HT203097

Things to try:

Plug your device into your computer to update using your computer.

Delete content from your device and try to update again.

Manage the storage on your device with recommendations.

If you can't update or restore your iPhone, iPad, or iPod touch - You can put your iOS device into recovery mode, then restore it using Music/iTunes.

https://support.apple.com/HT201263