Hi,
Just encountered same problem with Steam (Win 10) version so I tried install PCem emulator (it is real emulator not just virtualization) and installed Win 98 and original Jane's 688 + 1.07 patch and replayed first camp mission.
Just noticed in steam version UI screen backgrounds between missions are black and missing UI element.
This is how I got it in PCem with original CD version + 1.07
As you can see there is switch on right that I marked red and this one is used to continue.
In Steam version graphics is missing there but maybe area is still clickable. I not replayed yet with Steam version to confirm. Anyway if Sonalyst can get this fixed (what I do not hold breath for as it is ancient game) it will be nice.
Game works perfectly under PCem. Just you need set it up and install Win95/98 by yourself like on real old PC what is bit of work. I set CPU to Pentium 166Mhz and it is enough. Anything higher than 200Mhz is too much for my i7 as it is full CPU emulation.