I cant downgrade my storage plan

Restrictions as in Settings > ScreenTime > Content & Privacy Restrictions > Allow Changes > Account Changes - is this set to Allow?

If yes, have you tried restarting your iPhone? 

Do you have another device using your Apple ID that you can try downgrading the iCloud Plan as shown in https://support.apple.com/en-us/HT207594 ?

Is your iPhone connected to the internet? If connected via Cellular Data, have you tried using Wi-Fi and vice versa?

If you tried all steps, you will want to call Apple Support: HT201232

Hi!

What happens when you try to change your storage plan?

Did you try: On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Storage.
2. Tap Change Storage Plan.
3. Tap Downgrade Options and enter your Apple ID password.
4. Choose a different plan.
5. Tap Done.

HT207594

If it will not let you click on it, are you part of a Family Sharing plan or have Restrictions on your phone? HT201304