On the nRF52840 DK, P0.0 and P0.1 are connected to the 32KHz crystal: 32.768 kHz crystal. If you must use these pins, you need to short SB4 and SB3, cut SB1 and SB2, then change the LF clock source to the internal RC oscillator in your FW by adding CLOCK_CONTROL_NRF_K32SRC_RC=y to your prj.conf.
P0.26 and P0.27 should fine. Note that the silkscreen on the backside of the boards show which pins are being used by the board for LEDs, buttons, etc.
