NOTE: If you're having problem with the above form, you can donate directly on BitPay's site using this form set up for Pariyatti: Donate bitcoin to Pariyatti Donation Page.
If you're using Coinbase or a similar hosted wallet service then that could be the problem. In addition to not supporting Payment Protocol, Coinbase is not a Bitcoin or Bitcoin Cash Wallet. As their support page says, they are a “hosted wallet service.” This has pros and cons that BitPay outlines in its blog post on Wallets vs. Exchanges.