On Windows this error means that Reemo fails to capture your desktop, this might be due to a monitor beeing off or non-present.
In order to debug this issue, you could try to run reemod.exe from the command line. reemod.exe -debug_capture
On Linux this error usually means that the desktop is not using Xorg.
In order to debug this issue, you could try to run reemod from the terminal. sudo /opt/reemo/reemod -debug_capture