Debugger setup and usage

How to setup the debugger? I use a stm32 nucleo board (nucleo_f303k8) with stlink.
I get following error:


IDE 2.0.0-beta.2.2, Core 3.3.0, macOS sierra, atom 1.15
Another small thing, you can’t delete the first breakpoint you set.

See PIO Unified Debugger