I cannot add new debugger, unknown... Raspberry Pi 3B > /tools/arm-bcm2708/arm-rpi-4.9.3-linux-gnueabihf/bin/arm-linux-gnueabihf-gdb 
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[-]Can not add new debugger[/-][+]Cannot add new debugger[/+]carlonluca commentedon Sep 21, 2019
I had to install libncurses5 (sudo apt install libncurses5). Try to run gdb and see what error occurs.