I visit the subscription page, and click "Update". I click "Change Country" at the bottom, and select Thailand. Then I fill in my credit card details, and click "Change Payment Details."
After saving, I am redirected to the account page, but my country is still listed as the US. However, it did save my new credit card.
I visit the account page, and my plan is listed as Spotify Premium. I click "Change Plan", and then click the "Add Family" button next to Spotify Premium Family. Then I see a red error message notification at the top of the page, saying "Oops! Something went wrong, please try again."
Can you try this process using another browser or Incognito Mode? It might also be worth trying another device or a different internet connection. If you want to purchase a Premium subscription, you need to pay with a payment method that matches the country you're currently in. I'd also recommend checking out this Support article for more info.
Let me know how you get on!
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.