If I already Pau for extra iCloud storage on my iPhone 12 Pro, do I have to pay for it on my computer?

No. iCloud storage is linked to the Apple ID that you use for iCloud, not to specific devices. If you are using the same Apple ID for iCloud on your iPhone 12 and your computer, you should have access to the same iCloud storage.

MedHelpers1 Said:

"Currently I pay for extra iCloud storage on my iPhone12, do I have to pay a separate fee to have it on my MacBook Air?"

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Paying for iCloud on Separate Devices:

A. No. Once an iCloud Plan has been activated, it will be activated.

B. Verify the Purchase:

You can verify that the purchase went though. See this link: Manage subscriptions in the App Store on Mac - Apple Support

C. New iCloud Session:

If you can't see this upgrade anywhere, then log out of iCloud on all on your devices, and then log back in. Perhaps it all needs to re-sent?