Hola. No puedo instalar mi Mac a Monterrey. Me aparece que dice actualización no encontrada. ¿alguna idea?

Si tu Mac es anterior al 2015 no es compatible con macOS Monterey

Estas mac son compatibles con macOS Monterey , verifica si la tuya está en esa lista

MacBook Pro

Learn how to identify your MacBook Pro.

• MacBook Pro (16-inch, 2021)
• MacBook Pro (14-inch, 2021)
• MacBook Pro (13-inch, M1, 2020)
• MacBook Pro (13-inch, 2020, Two Thunderbolt 3 ports)
• MacBook Pro (13-inch, 2020, Four Thunderbolt 3 ports)
• MacBook Pro (16-inch, 2019)
• MacBook Pro (13-inch, 2019, Two Thunderbolt 3 ports)
• MacBook Pro (15-inch, 2019)
• MacBook Pro (13-inch, 2019, Four Thunderbolt 3 ports)
• MacBook Pro (15-inch, 2018)
• MacBook Pro (13-inch, 2018, Four Thunderbolt 3 ports)
• MacBook Pro (15-inch, 2017)
• MacBook Pro (13-inch, 2017, Four Thunderbolt 3 ports)
• MacBook Pro (13-inch, 2017, Two Thunderbolt 3 ports)
• MacBook Pro (15-inch, 2016)
• MacBook Pro (13-inch, 2016, Four Thunderbolt 3 ports)
• MacBook Pro (13-inch, 2016, Two Thunderbolt 3 ports)
• MacBook Pro (Retina, 15-inch, Mid 2015)
• MacBook Pro (Retina, 13-inch, Early 2015)

MacBook Air

Learn how to identify your MacBook Air.

• MacBook Air (M1, 2020)
• MacBook Air (Retina, 13-inch, 2020)
• MacBook Air (Retina, 13-inch, 2019)
• MacBook Air (Retina, 13-inch, 2018)
• MacBook Air (13-inch, 2017)
• MacBook Air (13-inch, Early 2015)
• MacBook Air (11-inch, Early 2015)

MacBook

Learn how to identify your MacBook.

• MacBook (Retina, 12-inch, 2017)
• MacBook (Retina, 12-inch, Early 2016)

Correcto no es compatible, solamente con macOS Big Sur fue la última versión, no te desanimes aún recibe actualizaciones por ahí viene macOS Big Sur 11.6.7