Hi,
You were almost right
You need to wrap individual columns in the format function, not the whole data frame:
asd = data.frame(a = format(c(4324322,45454354),big.mark=",",scientific=FALSE))
class(asd$a)
#> [1] "character"
asd
#> a
#> 1 4,324,322
#> 2 45,454,354
Created on 2020-07-29 by the reprex package (v0.3.0)
Hope this helps,
PJ