I can’t cancel iCloud storage plan to the free one. Currently I am on a paid plan of 200 GB.

You can "downgrade" your "200 GB" plan, marcela150.

However, if you have more than 5 GB used (the size of the Free plan), you will loose files!

Go to Settings -> Apple ID (the top of the lefthand listing, where you see your name) -> iCloud -> Manage Storage -> Change Storage Plan -> Downgrade Options (at the bottom of the dialogue box). (At that last, you'll be asked for your Apple ID Password.)

Cancel by downgrading to the free 5GB plan. See instructions in: https://support.apple.com/en-us/HT207594