  PeVatrons are the most powerful naturally occurring particle accelerators in
the Universe. The identification of counterparts associated to astrophysical
objects such as dying massive stars, molecular gas, star-forming regions, and
star clusters is essential to clarify the underlying nature of the PeV
emission, i.e., hadronic or leptonic. We present
$^{12,13}$CO(J=2$\rightarrow$1) observations made with the 1.85~m
radio-telescope of the Osaka Prefecture University toward the Cygnus OB7
molecular cloud, which contains the PeVatron candidate LHAASO J2108+5157. We
investigate the nature of the sub-PeV (gamma-ray) emission by studying the
nucleon density determined from the content of HI and H$_2$, derived from the
CO observations. In addition to MML[2017]4607, detected via the observations of
the optically thick $^{12}$CO(J=1$\rightarrow$0) emission, we infer the
presence of an optically thin molecular cloud, named [FKT-MC]2022, whose
angular size is 1.1$\pm$0.2$^{\circ}$. We propose this cloud as a new candidate
to produce the sub-PeV emission observed in LHAASO J2108+5157. Considering a
distance of 1.7 kpc, we estimate a nucleon (HI+H$_2$) density of 37$\pm$14
cm$^{-3}$, and a total nucleon mass(HI+H$_2$) of 1.5$\pm$0.6$\times$10$^4$
M$_{\odot}$. On the other hand, we confirm that Kronberger 82 is a molecular
clump with an angular size of 0.1$^{\circ}$, a nucleon density $\sim$ 10$^3$
cm$^{-3}$, and a mass $\sim$ 10$^3$ M$_{\odot}$. Although Kronberger 82 hosts
the physical conditions to produce the observed emission of LHAASO J2108+5157,
[FKT-MC]2022 is located closer to it, suggesting that the latter could be the
one associated to the sub-PeV emission. Under this scenario, our results favour
a hadronic origin for the emission.
