Diagnosis
The rapid flashing indicates the bulb is in pairing mode but failing to connect to your Wi-Fi network. The most common cause is an incorrect 2.4 GHz network selection.
How to Fix It
First, confirm your phone is connected to your 2.4 GHz Wi-Fi band, not the 5 GHz. Disable any mobile data on your phone during setup. In the Tapo app, ensure you are entering the exact password for the 2.4 GHz network. Reset the bulb by turning the physical switch off/on three times until it flashes rapidly again, then retry the 'Add Device' process.
If That Didn't Work
Your router's security protocol may be incompatible. Temporarily change your 2.4 GHz network security to WPA2-Personal (AES) only, not WPA3 or a mixed mode. Attempt pairing again, then revert security after success.
Heads Up
The bulb cannot connect if your router uses a 'Guest Network' or has 'AP Isolation' enabled. Ensure you are using the primary network.