Which country are you in? There are multiple different country-specific Amazon sites.

edit: The links are:

• Germany, Austria: https://www.amazon.de/dp/B07NJ6GZ5J
• Japan: https://www.amazon.co.jp/dp/B07NDNTDTP
• UK: https://www.amazon.co.uk/dp/B07NLBLZN2
• USA: https://www.amazon.com/dp/B01MT0UHLT
• Other countries: https://www.primevideo.com/detail/0MW6F85MD8486AE43GONNOR5F1

The Expanse is not available in Cuba, Iran, Mainland China, North Korea, and Syria, but one of the links above should work for all other regions and all 3 seasons should be included.

Make sure any VPN is disabled.

edit: Added Japan link. It couldn't be found by Amazon search with the English name.