At the time of writing, due to a packaging mess-up, the builds of SDR++ (and many other SDR tools by the looks of it) have suddenly and unintentionally dropped support for the popular RTL-SDR software defined radio dongle. Until this is fixed, users will have to build these applications themselves. The instructions are a little lacking in details, so here’s exactly how to do that, working on Kubuntu:

# To install:
sudo aptitude install cmake libfftw3-dev libglfw3-dev libvolk-dev zstd libairspy-dev librtlsdr-dev
git clone https://github.com/AlexandreRouma/SDRPlusPlus.git
cd SDRPlusPlus
mkdir build
cd build
cmake ..
make -j5 # 5 is the number of threads to use for building. adjust to your preference
sh ../create_root.sh
sudo make install

# To run:
sdrpp