Hello Osansa,
If your 50 GB storage plan is not showing when you go to Settings > your name > iCloud > Manage Account Storage (it's just showing the standard 5 GB plan), make sure you're currently signed into iCloud with the Apple ID that you have the storage plan under. If you have multiple Apple ID accounts for any reason, the storage plan may be set up under one of the other accounts.
If your 50 GB storage plan is showing there, but you have used all of the space you can use these recommendations if you're trying to clear things out of that space: Manage your iCloud storage
Or use these steps to change to a larger iCloud storage plan: Upgrade to iCloud+
You'll also want to make sure this notification you're seeing is actually for your iCloud storage plan and not the devices storage, which are two completely different things. If the notification is in fact about the devices storage, you can review what's using it in Settings > General > iPhone Storage.
How to check the storage on your iPhone and iPad
If you've used all of the storage space on your device, then you'll need to start deleting things on it to free up space.
Cheers!