Does it try to charge you $0? Sometimes happens with me too, but I'm able to get through just by the continue button. Or maybe you're serious? Try updating your store app, your phone OS, restart your phone, 'soft-reset' your phone, or anything else basic you can try. Make sure you're signed in with a Microsoft account on your device. Try using a different network (change to another wifi or sim network) Or like realwarder said use the 'Feedback Hub' to ask the developer about the issue, or you may directly contact the developer.
Although what app is it? If you give me its link maybe I can check the problem out.
Need Help? ask in the Comments. Hope this helps.