I recently updated my iPhone 12 to iOS 26. It seems there is a bug rendering Safari unable to "add to home screen" any website. Clicking the button displays a blank modal with Add button greyed out.
Our engineering teams need to investigate this issue, as resolution may involve changes to Apple's software. Please file a bug report, include a small Xcode project and some directions that can be used to reproduce the problem, and post the Feedback number here once you do. If you post the Feedback number here I'll check the status next time I do a sweep of forums posts where I've suggested bug reports.
Bug Reporting: How and Why? has tips on creating your bug report.