AWS Credits for billing

Unfortunately, you cannot use AWS Credits received this week to repay last month's bill. AWS bills are finalized at the end of each billing cycle, and once a bill is finalized, it cannot be updated or modified. This includes applying additional credits to your usage for a previous billing period.

AWS credits are applied automatically to future eligible charges on your account. They are typically used to offset costs for upcoming usage rather than retroactively applying to past bills. The credits you received this week will be applied to your current and future AWS usage charges, starting from the time they were added to your account.

To make the most of your AWS Credits in the future, it's best to ensure they are in place before the end of a billing cycle. This way, they can be applied to that month's usage before the bill is finalized. For your current situation, you'll need to pay last month's bill using your standard payment method, and your new credits will be used to reduce charges on your next bill.
Sources
Bill payment using credits added after bill generation | AWS re:Post
Applying AWS credits - AWS Billing