A few quick things to check:
Have you set up a passkey yet? The button only works after you've added one under Security Settings.
Is your phone's screen lock turned on? Passkeys need Face ID, Touch ID or a passcode to be enabled on the device.
Are you in a private or incognito window? Passkeys often won't work there. Try a normal window.
Did you open Sendana from inside another app, like an Instagram or WhatsApp link? Those built-in browsers can block passkeys. Open Sendana in Chrome or Safari instead.
If none of the above fixes it, tap the message icon at the bottom right of this page and we'll sort it out with you.