My storage is full .....

I was assuming from what you wrote that you have already used up your free 5GB of storage and were now being presented with a message about paying for a storage upgrade, in which you had no interest. Optimizing will free space on your phone but with any modern phone that has more than 8GB storage will likely require more than the free level of iCloud storage. Unless your phone storage is being taken up by things which do not require iCloud to store them such as hundreds of apps you use all the time and only a few photos, many people find optimizing entails an iCloud subscription.

What's the difference between device storage and iCloud storage? - "Learn the difference between your device and iCloud storage, ways to manage your space, and where to buy more if you run out." Click here --> https://support.apple.com/HT206504

Take-home:

1. Storage on a device itself is fixed and cannot be upgraded.

2. iCloud storage is on Apple's main computers. It is not a hard drive in the sky but by using optimization settings on your device it can help free up device storage. Lesser used items and larger file versions can be stored in ICloud and you can get access while connected to the Internet.

Click on this link to learn how to optimize the storage actually on the device itself --> "How to check the storage on your iPhone, iPad, and iPod touch" - https://support.apple.com/HT201656 - "If your device is low on storage, iOS automatically optimizes its available space while installing an app, updating iOS, downloading music, recording videos, and more. To make more storage available, iOS can remove some of your items, like streamed music and videos, files in iCloud Drive, and parts of apps that aren't needed. It also removes temporary files and clears the cache on your device. iOS only removes items that can be downloaded again or that aren't needed anymore."

You only option then is to delete files from your device. Transfer photos to your computer.

Transfer photos and videos from your iPhone, iPad, or iPod touch - https://support.apple.com/HT201302 - "Learn how to transfer photos and videos from your iPhone, iPad, or iPod touch to your Mac or PC. "

See the article Manage your iCloud storage - Apple Support

Delete the access data .