Can I use a Game Pass trial key on an existing account?

No. Trial keys are designed exclusively for new Microsoft accounts that have never had any subscription. If your account has ever had Game Pass, Xbox Live Gold, or any other subscription — even an expired free trial — the trial key will not work. You can check your subscription history at https://account.microsoft.com/services. If you want a key that works on any account, look for listings labeled "Standard" or "Renewal" instead of "Trial."