There’s a 3x limit to how smaller than video mode you can export the project, so at 240fps the limit is 240/3 = 80fps. To overcome this you can set the poject’s video mode to something smaller (like 60 fps, or just straight to 24) and then you’ll be able to export this.
I also had the same problem a while back but as long as there’s a workaround I was fine with it but I still disagree with the limit.