ios 15.8.4

That single-use code is usually what is called in English a verification code: “una código de verificación de uso único”, etc.

That code might be from an Apple account activity, or it might be a code received through another path such as via SMS, and then presented to the user in Safari.

If it’s not the Apple Account verification code, it’s probably this:

Rellenar automáticamente códigos de verificación en el iPhone - Soporte técnico de Apple (MX)

Automatically fill in verification codes on iPhone - Apple Support

And if that’s not it, yes, some more information on what is happening would be helpful; ¿Qué pasa?

¿Te refieres a esto?

Obtener un código de verificación e iniciar sesión con la autenticación de dos factores - Soporte técnico de Apple (US)

¿Qué pasa?

Automated translation from Spanish is “Enter the iOS 15.8.4 single use code”, which doesn’t make a whole lot of sense.

if, by any chance, you were asking about iOS updates, note that the iPhone 6s Plus listed in your tag line cannot update to iOS 16 or higher.