I am unable to buy iCloud storage even when I have money in iCloud balance. How do I buy the 200gb per month package?

See -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.

If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.

You pay monthly for iCloud storage.

See this Apple article: Upgrade to iCloud+

Upgrade to iCloud+ - Apple Support

Do you have another payment method on file? iCloud is a subscription, which requires a payment method to be on file.