Indeed, this journey will not be possible that day because of engineering works between Lelystad and Zwolle. It is a bit odd that the travel planner gives you this advice; it should know better (since it doesn't give this advice a week later).

You should travel via Amersfoort Centraal. In the travel planner, add Amersfoort Centraal as via station to obtain the following advice:
 

How is this possible NS. This is known for more than a month at the moment.

"It is a bit odd that the travel planner gives you this advice; it should know better (since it doesn't give this advice a week later).”

Thanks so much!

Do you know why I can't buy the ticket on the website or via the app when I pay for it?

Hey Andreapazs13. Welcome to the NS Community!

How odd, when I try to plan this on the site I can buy a ticket right there. Could you try this again on a different browser?

This photo is from this morning. I plan the trip and this is the result

Right. Is this still the result if you get if you plan it now? It shouldn't be.

You can't buy a ticket from within the travel planner if you plan a journey with a via station. You can buy a regular ticket from Emmen to Amsterdam Bijlmer ArenA without the via station. You'll be allowed to use it on the route via Amersfoort anyway since it’s the only reasonable route that day.

Word of caution: only buy the ticket now if you're absolutely sure you're going to make the trip. Etickets cannot be refunded. It might be wiser to buy it just a day in advance.