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!
As long you're not a US citizen, all you need to do is to submit form W-8BEN to prove you're not a resident and should not pay taxes in the US. If you this form hasn't issued to you by the company, you can find it on the IRS website or here alongside with other forms like w9 and else