How can I buy full version of software.

As the error message shows, contact iTunes Support for assistance. See the Apple Support articles below for contact information.   

iTunes Support – world wide 

Apple Support schedule phone call     

Billings and Subscriptions 

If necessary, try Twitter

https://twitter.com/AppleSupport.         

You may see various forms of a "Your purchase could not be completed" message. Here is the up to date URL for contacting Apple Support but it is not at all clear which path you should follow when you go to that web page. I assume this is it:

1. Click on this link --> https://support.apple.com/choose-country-region/media-services

2. Select your country

3. Select: Billing & Subscriptions

4. Select: Subscriptions & Purchases

5. Select: Update payment method

6. Select a chat session or a phone call.

Does it say why? We only know what you tell us.