Hey @firebuff101, I'll be happy to answer your question!
You won't be able to pay for your Premium subscription manually since Spotify automatically charges you at the end of your billing cycle. The reason is so that your Spotify Premium subscription can stay active without any interruptions.
If you don't have enough funds on your payment method, you can always change your payment details following these steps.
If you have any more questions, just let me know! I'll be here to help you 🙂