Using the pipe here, it is as if you're asking for
parse_datetime(dataset, date_maj)
If you read the doc, parse_datetime does not work that way with a data.frame as first argument.
You did not provide a reproductible example so I can't check, but this should work like that
parse_datetime(dataset$date_maj)
or
library(dplyr)
dataset %>% mutate(date_maj2 = parse_datetime(date_maj))
parse_datetime function works on vector only.
Hope it helps