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unable to purchase credit for my hobby plan

In what way are you unable to?

I don't see the add credit option on my hobby plan

Are you sure you're not on usage-based hobby plan?

You can see your subscription type here: (under Hobby Plan it'll either say credit-based plan or usage-based plan)

Yes am on hobby plan usage based subscription

And what that it means

You can't add credits to a usage-based plan unfortunately

How can I ensure that my account have enough credit from cancelation

Since it's usage-based it'll charge you automatically at the end of your billing cycle (monthly)

it's like a subscription except the price changes dynamically based on what you're hosting

!s