Ryugu can not be "geo"-synchronized or stationarized? With the known data of Ryugu asteroid, is therefore no way to be geosynchronized by any probe? The normal formula provides result non-physical. I explain myself...
With real data (at least, with known data), I get no sense of
$$R_\text{Ryugu-stationary}=\sqrt[3]{ \frac{G M_\text{Ryugu}T^2}{4\pi^2}}$$
where T is the rotation period of Ryugu, about 7,6 h. 
The mean radius of the Ryugu asteroid, about 850 m, is bigger than the radius of the "geo"-synchronized orbit, about 820m, since $R=radius+height$ is smaller than the "mean" radius of Ryugu, by a few tens of km. 
So, the height of the Ryugu-stationary probe would be negative...What is meaningless, since it is solid...
 A: There's a cute trick you can use to see that you've made a mistake, but I'll leave it to you to trace through your arithmetic to find where your mistake is.
It turns out that the orbital period for a "low" orbit, as defined below, depends only on the density of the primary.  You have the correct expression for the relation between semimajor axis $R$, period $T$, and primary mass $M$ for a low-mass object in a circular orbit.  Turn it around and solve for the period,
$$
T^2 = (2\pi)^2 \frac{R^3}{GM} \propto \frac{1}{G\rho},
$$
where $\rho \propto M/R^3$ describes the density $\rho$ if the semimajor axis $R$ is not very different from the radius of the primary.
You may be aware that Earth is made of rock and metal, with a mean density of about $5.5\,\rm g/cm^3$, and that satellites in low-Earth orbit have a period of about 90 minutes.  That means that any rock/metal object will also have surface-skimming orbits of about ninety minutes.  Even a super-low-density object like Saturn, with $\rho = 0.6\rm\,g/cm^3$, has surface-skimming (cloudtop-skimming?) orbits that are only about a factor of $\sqrt{5.5 / 0.6} \approx 3$ different from the periods of low-Earth orbits --- about 4.2 hours.
Since Ryugu has a density of about $1.1\rm\,g/cm^3$, its surface-skimming orbits are quicker than its seven-hour rotational period; therefore, "geo"-stationary orbits exist at some altitude.
