is there a reason why the following doesn't work:
> map_chr(.x = c("spades", "hearts", "clubs", "diamonds"), rep, 13)
Error: Result 1 is not a length 1 atomic vector
(ie, the code attempts to map the rep() function to the vector .x, passing 13 as ... to rep().)
but this does work:
map(.x = c("spades", "hearts", "clubs", "diamonds"), rep, 13) %>% unlist()
Based on the docs, I thought the whole point of the map_xxx functions was to return vectors.
In your first example using map_chr(), each operation returns a character vector of length 13, which is not a valid entry into a character vector. Only by catenating them or unlisting a list containing them (as you've done in the second example), can you get a character vector back.
What's the reason for using map() in the first place when it seems like you want rep(c("spades", "hearts", "clubs", "diamonds"), each = 13)? Or were you just casting about for an example?
map_chr() collect "a string element" from each iteration by function, then return the collect of the strings as list.
The function rep() does not returns an string element but returns a list of strings so, map_chr() makes an error.
Right, so rather than using map_chr() (which returns a character vector), you can use map() which will return a list. From the purrr cheat sheet 
It easier to answer question about a piece of code that doesn't work if you also include what you expect it to produce.
Just a guess but it looks like you want to produce 13 * 4 elements in your output, i.e. 52 the number of cards in deck.
But map and friends returns 1 output element for each element in the input so the map function you are using will return 4 elements in a list so you need an extra operation like this...
map(c("spades", "hearts", "clubs", "diamonds"), rep, 13) %>% flatten_chr(.)
This produces character vector with 13 * 4 + 52 elements in it.
This is an alternative syntax
map(c("spades", "hearts", "clubs", "diamonds"), ~ rep(.x, 13)) %>% flatten_chr(.)
which makes it easier to see what is happening with rep (just IMHO) because you need not know what map is doing with that "extra" argument 
If you are looking for some other kind of result just get back to us.
Casting about is certainly my usual MO. In this case, yes, I wanted a simple ex but think I outsmarted myself in the process. Thank you for the answer and for this terrific community site.