How do i uncheck "Do not fill empty party member slots" ?

[Bisceti]

I can't uncheck it, its just permanently checked.

[Apheirox]

I think it is permanently enabled for solo runs. Try switching to duo or triple runs and see if it can be changed.

[Crackless]

option is disabled when you go highroller.

[Icey Coldddd]

Make sure your in the correct team mode