Found a solution via the "lubricate" package from tidyverse:

co2_clean$Datetime = dmy_hms(co2_clean$Datetime)
    
co2_clean$hours = as.numeric(difftime(co2_clean$Datetime, co2_clean$Datetime[1], units = "hours"))

Still dont know what the problem is on my first approach, so if anyone could enlighten me on that I`d be very grateful!