You may be experiencing any of these issues:
Ensuring the SIM card is properly mounted in the phone, can solve some SIM activation issues.
During the initial setup, it is best to connect to a Wi-Fi network before starting the activation process.
Sometimes the phone does not connect properly to the mobile network service or network-specific apps after initial setup. Follow these steps to make the connection:
A factory data reset can also resolve some SIM activation issues. See How to factory reset a device.