If you have an iCloud subscription, you cannot remove your card because you need to have a payment method on file.
But you can change the card or change your payment methode.
See -> Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
You can also use your Apple ID balance as payment method: Add money to your Apple ID balance
Add money to your Apple ID balance – Apple Support (UK)
What you can buy with your Apple Gift Card or Apple ID balance
What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)
Your iCloud subscription:
Check -> Go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.
If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.
The subscription runs as long as you don't downgrade.