The problem is, as I understand it, that string theory (or, more generally, the theory that physicist call "M-theory") seems to allow a very large number of possible solutions, or as the physicists call them, vacuua (as in the plural of "vacuum"). In fact, there are roughly 10-to-the-500th-power vacuua. That's an immense number that I don't even know how to describe except by using scientific notation. It's much more than a googol, or even a googol of googols. But it's less than a googolplex.
It's about 10^109! (10-to-the-109-factorial). (Oops. Obviously not.)
Please note that the number of superstring theories given above is only a high-level classification; the actual number of mathematically distinct theories which are compatible with observation and would therefore have to be examined to find the one that correctly describes nature is currently believed to be at least 10^500 (a one with five hundred zeroes).