Is ios 15 is compatible with i phone 6s ? Cz my phone is downloaded the software but wheni i click on install its show an error occurred? Anyone can help please?

If your iPhone, iPad or iPod touch won’t update - Apple Support

Yes iOS 15 is compatible iPhone 6s. You were able to download it and that confirms that it is compatible. I see that you are having trouble installing it.

Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version-

• Your iPhone must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 6GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.