Looks like the problem is on the startup sequence somehow, so please try the one in the link below which goes direct to the game without showing the AmigaDOS screen with commands:
https://drive.google.com/file/d/17MFVn8So1YTvcAGp6bnoPKy6PrPis3gA/view?usp=shari...
You may have also CF card or similar hard disk, so you can also boot on Workbench, insert the floppy after Workbench has loaded. GreenBeret_FULL will appear on the screen. Drag and drop it to your Hard disk, let's say it's now copied to the Work: partition.
Now reset computer, hold both mouse buttons for boot menu, select Booth with no Startup. It'll bring you to the AmigaDOS screen, where you can type Work:
...then type cd GreenBeret_FULL
then type game.exe
It'll run from AmigaDOS, as a last resort if anything else works.
Also, a WHDload package has been released a few days ago: http://whdload.net/games/GreenBeret.html
So you can install and run from hard disk by clicking on Workbench icon instead of through AmigaDOS commands.