Refer to the following and you can resolve most setup issues.
A. Make sure the firmware of your Matter hub(Alexa, Siri, Google, SmartThings) and their respective app is updated to support Matter. This is the list of compatible devices.
B. Ensure your phone and Matter Controller are both connected to a stable 2.4 GHz Wi-Fi network, and that it has internet access during the setup process.
C. Power cycle the Matter-enabled device.
D. If the Matter-enabled device has already been added to another ecosystem, you will need to get a new setup code for pairing according to FAQ how to integrate your device into another ecosystem.
E. In the first attempt, Matter pairing mode is active for 15 minutes after the device is powered on. For the second set up, the active time of setup mode varies between ecosystems.
If this time period has expired, you will need to power cycle the matter-enabled device to restart the clock.
F. Restart your smartphone and clear the cache of the Smart Home app