The application is very young, it is not unlikely that you encounter problems with it.
Here are the common reported problems and their status.
The most common is your device not being detected by adb, or is unauthorized.
Check everything is ok by calling:
adb devices
Windows may need some drivers to detect your device.
On some devices, you may need to enable an option to allow simulating input.
On MacOS, with HiDPI support and multiple screens, input location are wrongly scaled. See issue 15.
A workaround is to build with HiDPI support disabled:
meson x --buildtype release -Dhidpi_support=falseHowever, the video will be displayed at lower resolution.
On Plasma Desktop, compositor is disabled while scrcpy is running.
As a workaround, disable "Block compositing".