From this page, it looks like Stripe is available in Hong Kong: https://stripe.com/global
Whether or not you need to pay tax on your Thinkific orders really depends on what you're selling, where you're selling from, and where you're selling to. I'd recommend checking with a local tax expert for some help with that, as we're not able to provide definitive tax advice. Once you know, here's some information on how to handle tax on Thinkific.
I hope that helps!