  Comparing chemical abundances of a planet and the host star reveals the
origin and formation path. Stellar abundance is measured with high-resolution
spectroscopy. Planet abundance, on the other hand, is usually inferred from
low-resolution data. For directly imaged exoplanets, the data are available
from a slew of high-contrast imaging/spectroscopy instruments. Here, we study
the chemical abundance of HR 8799 and its planet c. We measure stellar
abundance using LBT/PEPSI (R=120,000) and archival HARPS data: stellar [C/H],
[O/H], and C/O are 0.11$\pm$0.12, 0.12$\pm$0.14, and 0.54$^{+0.12}_{-0.09}$,
all consistent with solar values. We conduct atmospheric retrieval using newly
obtained Subaru/CHARIS data together with archival Gemini/GPI and Keck/OSIRIS
data. We model the planet spectrum with petitRADTRANS and conduct retrieval
using PyMultiNest. Retrieved planetary abundance can vary by $\sim$0.5 dex,
from sub-stellar to stellar C and O abundances. The variation depends on
whether strong priors are chosen to ensure a reasonable planet mass. Moreover,
comparison with previous works also reveals inconsistency in abundance
measurements. We discuss potential issues that can cause the inconsistency,
e.g., systematics in individual data sets and different assumptions in the
physics and chemistry in retrieval. We conclude that no robust retrieval can be
obtained unless the issues are fully resolved.
