I'm pretty sure that means your drive is too full to add a partition.
Make more space. Going by your figures, I suggest that you remove enough to make the Windows partition in the size that you think you may need. Seems that the system is counting some space that appears to be free, but is not, such as space that might be reserved for files that are actually stored in your iCloud drive. The system reserves that space, because iCloud files are listed in the directory, but not actually stored locally. That's my take on how that works.
So, thinking that might be the situation, add the amount that you want to use for Windows to that free space, then remove that amount of data/files that will give back that space. Just a little math shows that (for an 86GB boot camp partition) you should remove (copy to another drive, then erase/delete from your boot drive, so the space is released) about 76 GB to have enough free space on the Mac partition. I would probably add 25% to that, and remove a total of 95 GB (round that up to 100 GB). That looks almost like the amount that I used in post #5. Still sounds good to me, but --- does that make sense to you?