I need to but something on my phone with my debit card but is not working whyv

Looks like you have to check your payment method, see the following Apple articles:

If you can’t download or update apps on your iPhone or iPad

https://support.apple.com/en-gb/HT208687

Change, add or remove Apple ID payment methods

https://support.apple.com/en-gb/HT208687

Add money to your Apple ID balance

https://support.apple.com/en-gb/HT208687

Turn VPN Off & use Wi-Fi network.