How do I concatenate two IntStreams?

You're passing to concat two Stream<IntStream>, which won't work (you want a stream of integers). You need to give it two Stream<Integer>:

List<String> strings72to200 = Stream
        .concat(IntStream.range(72, 129).boxed(), 
                IntStream.range(132, 200).boxed())
        .map(String::valueOf)
        .collect(Collectors.toList());

And just a side note, if you intend to include 129 and 200 in the streams, you should use IntStream.rangeClosed (end is exclusive)

You might just be looking for boxed there to get a Stream<Integer> and then concatenate :

List<String> strings72to200 = Stream.concat(
            IntStream.range(72, 129).boxed(), // to convert to Stream<Integer>
            IntStream.range(132, 200).boxed())
        .map(String::valueOf) // replaced with method reference 
        .collect(Collectors.toList());

Edit: If you were to just get an IntStream from the given inputs, you could have concatenated them as:

IntStream concatenated =  Stream.of(
        IntStream.range(72, 129),
        IntStream.range(132, 200))
    .flatMapToInt(Function.identity());

or simply

IntStream concatenated =  IntStream.concat(
        IntStream.range(72, 129),
        IntStream.range(132, 200));