why can't i upgrade to 50gb when i click on up

See this Apple article: Upgrade to iCloud+. -> Upgrade to iCloud+ - Apple Support

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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+. 

If you have upgraded iCloud storage, it would take about 1 to 3 business days in order to process.

Once your request for iCloud+ has been activated, you should receive an invoice in your email box.

Christina_30 Said:

"Why isn’t there an option to upgrade to 50gb?"

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For reference...

Troubleshooting iPhone Storage:

Internal storage cannot be upgraded. It is what it is. So, the maximum internal storage capacity never increases, even if you upgraded your iCloud plan. Learn More here: What's the Difference Between Device Storage and iCloud Storage? Apple Support. 

So, what to do:

Clear Space:

Having little empty space can heat things up. So, here is what to perform...

A. Clear Temporary Internet Files:

Clear your Temporary Internet Files(Cookies & Cache) of Safari:

• Cookies perform remembrance of certain site-entered data.
• Cache keeps a snapshot of the last time you visited a site.
• Go Here: Clear the History and Cookies from Safari on your iPhone, iPad, or iPod Touch - Apple Support

Also...

B. Clean TV and Music Libraries:

Remove all unneeded files from your TV and Music Libraries. Full-Movies take up 2-4GB typically, and a bunch of those would take up a load of storage, ideally. So, if you have any movies that are not needed, then delete them.

Also...

C. Clear "Other":

Clear the storage named System Data (formerly called "Other"). It basically contains unnecessary files such as caches that are used for system ease of use - all of which gather over time. Perform as I've instructed here: How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip. Caches would be being used for remembrance of application use.