This error means that one of your homing switches on the tandem axis is triggered, but the other one is not.

For safety reasons, the software will not proceed with tandem homing as this can mean that one of your homing switches is broken.
If your switches are functional and there is no issue, the machine will have to be jogged away from the homing switches to proceed with homing.

If your machine is set up in such a way that one switch is often triggered but other one is not - for example right after homing - you can add a move in your homing subroutine in the macro file to prevent this state.