please i want to purchase storage but don't have credit

but don’t have credit card.

You may need to have a payment method on file, even if the app you want to download is free. 

Check which payment method you have on file 

See the following Apple articles:

Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)