I cant buy with my iTunes card

See If your payment method is declined in the App Store or iTunes Store https://support.apple.com/en-us/HT203005. If you need additional help, use this support link from the document https://support.apple.com/contact

But, also see these reads pertaining to a related error: 

Payment method declined in iTunes or the App Store ...

Fix 'Your Payment Method Was Declined' Error In App Store

Why do I keep getting a message that my Apple payment ...

Also, can get Apple telephone support  for help in solving your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362  Be patient sometimes Apple support is closed to immediate phone support.

or use Apple Support Twitter https://twitter.com/AppleSupport

And here is how to contact iTunes worldwide support via text or chat using this link https://support.apple.com/choose-country-region/itunes After choosing your country, see the heading Tell us how we can help.

As the screen shot shows, contact iTunes Support. See the Apple Support articles below for contact information.   

iTunes Support – world wide 

Apple Support schedule phone call       

If necessary, try Twitter

https://twitter.com/AppleSupport.          People have posted it may take an hour or more to get a response.