Why I can’t do ios 15?

If your Apple mobile device won't update, read this support article --> If your iPhone or iPad won't update - Apple Support

Another, less common reason for not being able to update is if you were using a beta version of software. Refer to How to uninstall iOS or iPadOS beta software - Apple Support

Are you sure you have an iPhone 6s and not an iPhone 6? Go to Settings>General>About>Model Number. What does it say?