"What level of beginner can/should start NSM?"
I have this question myself, and I tried to find answers in the book/reddit threads but didn't find a definitive answer. I'm posting this in the form of an answer to summarize what I have found and show that I wasn't being lazy. BUT I am not an NSM expert and I will accept any corrections.
TLDR: If you are not running at least 4.5 hours a week, spread across 6 days of the week, you are not the kind of "beginner" that people in these spaces are talking about.
What is NSM?
Briefly, because the OPs of other threads asking this question dont even seem to know:
The Norwegian Singles Method is a training structure targeted at runners spending 6-8 hours per week on running. The canonical form also assumes you run everyday of the week.
The "selling point" of NSM is that it will allow you to make the most of this "limited" running time, while allowing you to repeat the same structure (with minor variations), week after week with no "boom and bust" cycles.
Is it for beginners?
Some people will say yes, but in running, "beginner" is a very broad term.
I suspect this has to do with the recent rise in the popularity of running. Some people think 15 miles a week is "solid", while experts in the field are making videos like "How to get good while running 40 miles a week" becuase 40 is considered low. But I digress
NSM is made to make the most out of a "limited" 6-8 hours per week. For perspective, a C25K finisher is running 1.5 hours per week, of all easy volume. NSM is targeted at people running 4x that volume and are physically capable of running more, but cannot due to logistics/desire rather than durability.
In the book (link removed because that seems to be a problem), there is no "are you ready for NSM?" checklist. There is a 4.5 hour / week schedule, that he describes as a "gateway" to canonical NSM. That routine has 3 speed sessions, and 3 easy runs, one of which is a long easy run.
Answer: NSM is not for "beginners" in the colloquial sense of the word "beginner". Its for serious novices
If you aren't running at least 4.5 hours per week, on 6 runs per week, you need to work up to that first to even begin incorporating the 2.5 speed sessions described in the "gateway" plan.
You need to be advanced enough to look at 4 consecutive days where you do a Speed^(tm) day, a long run day, an easy run day, and yet another speed day and consider that "easy" day the recovery period between your long run and the next speed day.
If you are a true beginner who fell into the NSM discussions, you should aim to get to 6 easy runs per week, including an easy long run. And then incorporate "half of a speed day" as described in the gateway plan of the book into one of the runs out of 6. And then turn 3 of the 6 days into those "half speed" days, and then begin turning those 3 days into the real sub threshold speed days.
A lot of the threads you see amongst novice/intermediate runners in spaces discussing NSM are amongst people who ran out of time in the week to do "standard" polarized training.