I have the nRF52805 device now and I see what you see in the debugger.

This code will fail in the debugger, since sd_power_system_off will return with error in debug mode. In debugger mode, the systemoff is emulated and I think instead of APP_ERROR_CHECK, you should have a while(1); here. The error is expected and a correct behavior, but i can see that it causes some confusion.
I recommend you to replace the last APP_ERROR_CHECK(err_code); with while(1);

Thanks for your help, I replace the last APP_ERROR_CHECK(err_code); with while(1);The reset happens when your chip wakes up from the system off sleep. Seems like you are configuring the wrong GPIO pin number as the wakeup button source. Which board file are you using in your project from the nRF52805_sleep_main\components\boards directory. I think the button connected on your board is connected to a different GPIO than the board you included causing a wrong wakeup source for your chip. which in turn is waking your chip immediately after going to sleep which causes a system reset.
The reset happens when your chip wakes up from the system off sleep. Seems like you are configuring the wrong GPIO pin number as the wakeup button source. Which board file are you using in your project from the nRF52805_sleep_main\components\boards directory. I think the button connected on your board is connected to a different GPIO than the board you included causing a wrong wakeup source for your chip. which in turn is waking your chip immediately after going to sleep which causes a system reset.