I went to the link you posted and grabbed a copy of NVFlash for Linux as listed. It can wrapped in a .tar file, which I extracted.
The extraction provided an NvFlash program as a standalone program x-exectuable type, not an .exe
Can you please double check your download and source?
If that is still not working for you, installing it from source may work out better for you:
https://ac100.grandou.net/nvflash
I recommend running any commands to flash the GPU while in TTY so that you are not actively using the Nvidia card when flashing it.
Then unload the modules:
rmmod nvidia_uvm nvidia_drm nvidia_modeset nvidia
Once completed, reload the modules:
modprobe nvidia_uvm nvidia_drm nvidia_modeset nvidia
Warning Note:
Directly flashing the Nvidia GPU is a viable option for some cases, but it always carries an element of risk. I highly recommend backing up all your personal files and data before proceeding. Only flash the GPU if it is necessary due to the age of the card causing problems that flashing will correct. This is not a "regular update" that should be performed as a routine maintenance action. On some older cards, it may cause failure of the card.