different systems have different conventions around path delimiters, its generally advisable to have R construct a path for you by using file.path()
perhaps
writexl::write_xlsx(decstats,
file.path("C:","Users","Tommy Mulflur","Desktop","decstats.xlsx"))
also if theres any doubt about the directory , you can check if R undserstands it exists with something like
dir.exists(file.path("c:","Users"))
dir.exists(file.path("c:","Users","Tommy Mulflur"))
maybe adding it up iteratively will show you if you have a typo or somesuch along the way