The micro switch is inside the lock mechanism. Is this a motorized door? If not, check out the switch: I use a phillips screwdriver as it has a round barrel. The latch has 2 positions, push the latch to the first position, turn the ignition to posII and the warning message should be there. Push the latch to completely locked, the message should go away. Use the handle to unlock the latch.

Adjustment might be necessary, or just cleaning out the latch at the top of the bumper.

If your tailgate slams shut, you need to get new struts.
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My back feels better when I sit in a Volvo seat