Hi @Selemani_Ngwira, welcome!
For get a better help try to put a reproducible example of the data and script.
See a tentative example for change specific columns with NAs to Zero.
Covid_19_Vaccination <- data.frame(
Country = c("A", "B", "C"),
Col1 = c(10, NA, 20),
Col2 = c(30, 40, NA),
Col3 = c(NA, 50, 60))
Covid_19_Vaccination <- Covid_19_Vaccination %>%
mutate(across(1:2, ~ifelse(is.na(.), 0, .))) # Select 1 and 2 columns
# Country Col1 Col2 Col3
#1 A 10 30 NA
#2 B 0 40 50
#3 C 20 NA 60