Hi Karim_12,
If you haven't done so yet, restart your iPhone and try again. Sometimes, restarting can correct unexpected behavior. If the issue persists, try connecting to a different network. This will let you know if the issue was possibly caused by the network you were connected to.
Also, make sure you're following these steps to upgrade: Upgrade to iCloud+
How to upgrade on your iPhone, iPad, or iPod touch
1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan and follow the onscreen instructions.
If you're on your iPhone, iPad, or iPod touch with iOS 14 or iPadOS 14 or later, tap here to upgrade to iCloud+.
Best regards.