It is impossible to buy tickets for the Olympic Games trough the official site of Tokyo 2020. Only residents of Japan can use that site to buy their tickets.
Every country has it’s own official Authorized Ticket Reseller. Herewith a small overview. It is recommendable to subscribe on their newsletter. That way you will be informed from the moment tickets are available (JUNE)!
Netherlands: SUNWEB SPORTS & EVENTS
Germany: DER TOURISTIK DEUTSCHLAND
For other countries: see official ATR list