You can read in your data from Excel using the readxl package.
As you haven't provided a reproducible example, I have mocked up some dummy data. This achieves what you want I believe:
library(tidyverse)
df = tribble(
~listing, ~title, ~sku1, ~sku2, ~sku3,
101, "necklace", "abc123", "abc1234", "abc12345",
102, "earring", "abc239", "abc293", NA,
103, "bracelet", "abc398", "abc308", "abc183"
)
df
#> # A tibble: 3 x 5
#> listing title sku1 sku2 sku3
#> <dbl> <chr> <chr> <chr> <chr>
#> 1 101 necklace abc123 abc1234 abc12345
#> 2 102 earring abc239 abc293 <NA>
#> 3 103 bracelet abc398 abc308 abc183
df %>%
pivot_longer(contains("sku"), values_to = "sku") %>% # make longer
select(-name) %>% # get rid of the name column
drop_na(sku) # get rid of missing values
#> # A tibble: 8 x 3
#> listing title sku
#> <dbl> <chr> <chr>
#> 1 101 necklace abc123
#> 2 101 necklace abc1234
#> 3 101 necklace abc12345
#> 4 102 earring abc239
#> 5 102 earring abc293
#> 6 103 bracelet abc398
#> 7 103 bracelet abc308
#> 8 103 bracelet abc183
Created on 2022-01-16 by the reprex package (v2.0.1)