To enable the low-frequency clock (LFCLK) in the nRF52840, you have two main options: using an external 32.768 kHz crystal oscillator (connected to P0.00 and P0.01, which is used by default) or using the internal RC oscillator. I suppose you want to use the internal RC oscillator.. For this you need to add the Kconfig CONFIG_CLOCK_CONTROL_NRF_K32SRC_RC=y to prj.conf.
