"vandenberg p" wrote:
Why? For the same reason you will not be able to find the
value of X in the following: 6X^2 - 10X + 5 = 0[.]
Look up Descartes rule of signs. There are 6 sign changes.
Therefore the solution may have as many as 6 answers or
as few as zero.

I do not believe that necessarily explains the #NUM errors.
IRR has no trouble computing the rate (2%) of the following
cash flow, despite 8 sign changes:

-100000
{10000,-1000} eight times
53435