I’m trying to get the new iOS 15 update and it won’t let me

To update 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
• The phone cannot be in Airplane mode

Depending on what type of iPhone you use.

If your iPhone is eligible for an update, you can check by reading this Apple article:

iPhone models compatible with iOS 15

https://support.apple.com/en-gb/guide/iphone/iphe3fa5df43/ios

What happens when you go to Settings/General/Software Update?