How can I use XL0 and XL1 as UART pin

the pin for XL0 and XL1 is P00 and P01

but in my project, I need P00 and P01 works as UART.

I set the parameter to start inner RC clock.and find that:  LFCLKSTAT show the src is    0 (32.768 kHz RC oscillator)  and status is working.

the overlay file shows the uart pin:
uart0_default: uart0_default {
        group1 {
            psels = <NRF_PSEL(UART_TX, 0, 1)>,
anyway, the uart cannot find signals. 
If I set uart tx to P06. the uart string can be find. 
Please tell me how can I allocate the UART pins to P00 and P01. 
thanks a lot. 
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