Why can’t my iPhone 6plus get an update

An iPhone 6 Plus can only run as high as iOS 12.5.5

iOS 15 compatible devices --> https://support.apple.com/HT212712

Here's a simple and fast way to see what is possible with your device, including updating it to the highest sub-version (e.g., 14.7 --> 14.8) of an iOS/iPadOS you may already have:

On your device, tap:

Settings > General > Software Update.

1. If it says your device is up to date there is nothing more you can do. If apps you are trying to use say you need a higher iOS version, you will need to buy a newer device if you want to run those apps or features.

2. If it says an update is available you will normally only be given one update selection which will be the highest available iOS version supported by your device. However, if you are running iOS 14.5 or greater, or iOS 15 you may see two options. Click on a link below for full instructions about how to update wirelessly, or using a computer (see links in document) if you have storage or connectivity issues. If the update listed is not equal to or higher than the iOS version you need to use a feature, you again will need to buy newer equipment if you want to use that feature.

Update your iPhone, iPad, or iPod touch. Click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY