Ok, so you actually have 573kbytes free - which isn't brilliant, but not horrendous either. Ignore what the game tells you - we don't know how it is calculating that.
So you can free up some more of that 640kbytes by adding himem.sys and loading DOS high, which is perfectly possible on a 286.
Add to your config.sys, at the very start:
That should take you to 600kbytes free, or more.
DOS memory management is an art/science in itself. There are some helpful tips here: Dos 6 conventional memory tricks
But, things to note on a 286:
- You don't have EMM386 or EMS in general - unless (a) you use the very slow EMM286, (b) have a very specific 286 chipset which supports it, or (c) buy a hardware EMS ISA card (you may or may not need EMS memory depending on which games you want to play).
- You don't get UMB support to load drivers/DOS components out of 640Kbytes - unless your chipset is on the compatability list of a memory manager like (a) 'The Last Byte' or (b) 'UMB_DRVR'
BTW, What version of DOS are you using? I'd not really want to use anything earlier than DOS 5. Other than being slightly larger on-disk and taking up a small handful of kbytes more in-ram, there's really no reason (other than nostalgia) to use anything other than 6.22.
My collection database and technical wiki: