While searching for flights on Google Flights, I encountered an entry for a major airline that specifies "Call [airline] to book" and lists a phone number, rather than providing a direct "Book with airline" link. Interestingly, third-party OTAs (like Expedia and Flightnetwork) are able to price and sell the flight online. Why would Google Flights show "Call airline to book" for a major carrier instead of a website link?
Some airlines enforce more strict rules on their website than they do via other sales channels - especially when it comes to combining different fares and/or flying partner airlines.
It is not uncommon for Google Flights to find/offer specific combinations of flights that the airline's website will not display if you were to carry out the exact same search. For example, the airline might decided not to show any flights with particularly long connections (especially >24 hour connections) in their flight search results - where Google Flights does frequently show such results.
Some airlines provide Google Flights with a means to link to their website for such a booking - allowing you to book a combination of flights on the airline's website even where the website wouldn't otherwise let you create such a booking. Other airlines do not provide such a mechanism - and as you've discovered Thai Airways seems to be one such airline.
These flights will be bookable via other travel agencies (as per the list shown in your results), and will likely also be available via the airline if you contact them directly rather than going via the website - which is exactly what Google is suggesting you do.
I had exactly the same issue with a flight combination I was looking at on Thai Airways just over a week ago. In that case, the flights included a partner airline (Virgin Australia), and their website would only allow me to book a specific partner connecting flight, and not the flight an hour earlier that was cheaper. Google Flights found the cheaper flight, and gave the option to book via other websites as per your example - but suggested I had to call to get that combination on Thai.
