I've been receiving messages telling me that my iCloud storage is full and thus not baked up. I've been paying $.99 since 2018 for my iPhone 8. Do I need to pay another $.99 to upgrade the storage capacity? Please advise me what to do.

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Buy more iCloud storage

You automatically get 5GB of free iCloud storage for your photos, videos, files, and more. If you need more iCloud storage, you can upgrade from any of your devices.

https://support.apple.com/en-us/HT201318

iCloud storage works like this: You get 5GB free, the next level is 50GB for $0.99/mo, then comes 200GB for $2.99/mo, then 2TB for $9.99/mo. If you have reached your 50GB limit then your next option is $2.99 for 200GB. Of course another option is to downgrade back to the free tier but you will have to delete enough data to get back to the 5GB.