Understanding why this error happens is the first step toward fixing it:
The official Activision master servers are largely inactive, leading to verification failures.
Wait for the "Awaiting Key Code Authorization" message to appear.
Open the game console by pressing the key (usually next to the "1"). Type /reconnect and press Enter.
Multiple active network adapters (like Wi-Fi and Ethernet) or VPN virtual adapters (e.g., Hamachi) can confuse the game's communication.