GPU Memory Allocation option not selectable in "Pi Config" settings

I want to change the GPU Memory on my Pi 4, and since it has 4gb RAM I decided to change it to 512mb, but the Memory Allocation option in the Pi Config is greyed out and unselectable. I tried changing config.txt with gpu_mem, but all values prevented OSMC from starting properly.
I’ve read that the Pi 4 uses automatic memory allocation to the GPU, but I still get messages from programs telling me that I “have less than 128mb of GPU Memory”.

I don’t remember the details exactly, but you don’t want, or need to, change the memory split on a RPi 4 as it doesn’t use the same type of setup as earlier Pi’s. If memory serves the new Pi hardware uses dynamically allocated space for the GPU and if you manually set the split outside of certain sizes it breaks certain functions.