I've actually a small leaflet, which describes the error.
Excellent! I found some copies of the game manual online, but none seem to have any pages devoted to troubleshooting errors. Curious where you got the leaflet from?
Still stand by my complaint that the error message is so badly designed. It prints out all those technical details yet fails to say which kind(s) of memory is too low.
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The problem is that the game requires at least 570kb (601kb /w sound) of main memory
That's rather high! As I recall, since DOS runs in x86 real mode, "main memory" (aka "conventional memory") is limited to 640kb, and some portion of it will always be in use by DOS itself. So I guess Lemmings 2 with sound would need nearly all the remaining. Other kinds of memory goes into the "expanded" or "extended" type of memory, and are treated specifically because they require extra work to make use of by real-mode DOS programs.
I suggest running the "mem" command Forestidia mentioned and get its output, to see more details on memory usage to maybe better understand why your computer don't seem to have the full 640kb typically available. Perhaps you have extra device drivers and/or background services running that can be disabled or otherwise re-configured to not eat into the 640kb? I suppose it's also theoretically possible that if some memory chip gone bad it might have happened to cut off the full 640kb available, though that seems a bit of a stretch.