Adding Rows After Missing Sequence [duplicate]

You could use complete to fill the missing days.

library(dplyr)
library(tidyr)

df1 <- df %>%
        mutate(DAY1 = DAY) %>%
        complete(ID, DAY, fill = list(DAY1 = -99)) %>%
        select(ID, DAY = DAY1)
df1
#      ID   DAY
#   <int> <dbl>
# 1     1     1
# 2     1     2
# 3     1     3
# 4     1     4
# 5     2     1
# 6     2   -99
# 7     2     3
# 8     2     4
# 9     3     1
#10     3   -99
#11     3     3
#12     3   -99

For the updated data we can try to add day by group :

df1 <- df %>%
        group_by(id) %>%
        complete(day = min(day):max(day), fill = list(amount = -99)) 
df1

data

df <- structure(list(ID = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L), DAY = c(1L, 
2L, 3L, 4L, 1L, 3L, 4L, 1L, 3L)), class = "data.frame", row.names = c(NA, -9L))