I am getting the same error message - however - this is on a 2013 Mac Pro - The problem with this error is that the Apple dialog does not tell us WHY? So, on macOS High Sierra, I did the following to find more information as to why it might be happening:
1). open a terminal, make it as wide as possible
2) enter the following command to watch the logs go by in that window: "sudo log stream"
don't include the quotes above, I think you have to use sudo for permissions
3) try to get Boot Camp Assistant to try again. When it has the error, look for errors relating to partition creation or resizing.
In my case, I get the following errors - I do not know how to fix it yet but it is apparently related to APFS:
2017-12-02 20:36:54.799131+0000 0x1855a Default 0x0 0 kernel: (apfs) spaceman_is_block_out_range_free:5703: Free count in blocked-out range 52055711, sm objects size 0, out of 52055712
2017-12-02 20:36:54.799157+0000 0x1855a Default 0x0 0 kernel: (apfs) nx_block_out_physical_range_internal:2985: Blocked out range is not free after eviction
2017-12-02 20:36:54.799162+0000 0x1855a Default 0x0 0 kernel: (apfs) nx_block_out_physical_range_internal:3067: nx_block_out_physical_range failed with error 35
2017-12-02 20:36:54.801816+0000 0x1855a Default 0x0 0 kernel: (apfs) nx_resize:3461: block_out_physical_range failed, error 35
2017-12-02 14:36:54.815639-0600 0x18518 Default 0x2e0f0 1944 Boot Camp Assistant: (DiskManagement) DiskManagement: request for localized user error string for code -69606
2017-12-02 14:36:54.815684-0600 0x18518 Default 0x2e0f0 1944 Boot Camp Assistant: BCA: partition Error <A problem occurred while resizing APFS Container structures.> (-69606)
This doesn't solve the problem for me yet, but at least I have some information to go on.