Most times, promo codes cannot be applied to your account because your account doesn’t comply with Google’s requirements.
If the issue is that you haven’t entered a valid payment method, we highly suggest you do. You'll need to enter valid payment information in order to activate your Google Ads account and get your promotional credit. This is to ensure that new accounts are created purposefully. It also ensures that your account is set to go with valid payment information, in case you decide to continue advertising after you've used your credit.
If the issue is that your account is not new or is inactive, we can try to create a new Google Ads account for you so you can receive the credit (although this is not always successful).
If all else fails, here is a resource for you to trouble shoot promo code issues directly with Google.