I don’t know what to do to allow my iOS 15 to install

What iOS is currently on your iPhone? If you really have an iPhone 6s as indicated in your profile, it should support iOS 15.x. If you go to Settings>General>Software Update, what does the message say? If it shows you are currently on iOS 12.5.5 and that you are up to date, then you have an iPhone 6, not 6s and it does not have the ability to update. The hardware will not support the newer iOS.

If you do have an iPhone 6s, then see the instructions here to update your device. Update your iPhone, iPad, or iPod touch - Apple Support

To update over the air from the phone all of these requirements must be met:

• You need at least 6 GB of free storage
• You must be connected to a Wi-Fi network (and not a personal hotspot)
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN, and it may be necessary to delete any VPN apps and VPN profiles
• The phone cannot be in Airplane mode
• You cannot still have a beta version installed on the phone<—This is a likely suspect
• You cannot have a previous version that was downloaded, but not installed - go to Settings/General/iPhone Storage to look; if you find an iOS installer, delete it

If you can't meet all of them you can still update using your computer.