If you've not yet got an extra relay, how are you connected the v3 relay?
Are you connecting it so the spindle only works in one direction (i.e. only applying power to one side of the contactor) when active, or as a changeover so there's always power to one of the contactor coils?

If you're going for the one direction, then you need to assign the Relay output # to whatever pin you have the v3 dir terminal connected to, then set the dir pin in Motor Outputs to something that's not being used.

If you're going for the permanent live option, then you'll need to disable the spindle relays, and set Spindle dir to whatever pin you have the v3 dir terminal connected to. Then to activate the relay for testing, either tell mach to start the spindle in reverse, or tick Dir Low Active for the spindle and start it forward.