My payment is declined, and a $20 charge occurs when installing apps

so whenever i go to my icloud storage, it says i’ve used 37.9 out of 50 gb. but whenever i try to install a 1 gb app, it says my payment method was declined, and then it try’s to charge me $20 on my card. does anyone know what this is?

[Re-Titled by Moderator]

Original Title: not allowing me to install apps