INAV configuration for Spektrum Radios

 I don't have any Spektrum gear to check this but INAV setup is the same as any other radio system apart from channel order and Serial Receiver Provider.

Spektrum radios use TAER channel order and Serial Receiver Provider is SRXL2 (or SBUS if available). Make these selections in the INAV Receiver page. 

SRXL2 signal wire connects to the T pin of your selected Serial Receiver UART.

Configure the INAV Model Mix in the radio with 100% for the first 4 channels (TAER), and the Arming and Mode switches. This may be the Multirotor premix in Spektrum radios.

In the INAV Receiver page, check that the channel graph bars move to the right when you move the sticks up and right.

If all this is working but Arming is still not possible check this Troubleshooting checklist