Boot camp assistant gives "Windows support software could not be saved to selected drive" Error Installing Win 10 on MBP2015
I will address your attempt to manually install Windows 10 without the use of the Boot Camp Assistant. Here are some points.


You are using Sierra. This is good in the sense that High Sierra has introduced APFS. This has caused some problems that you will be avoiding.
You created a bootable USB installer. I do not know how you did this, but you do have to copy the Windows Support software to this USB flash drive before installing Windows. If you can not do this, let me know.
I appears you created a FAT formatted fourth partition labelled "BOOTCAMP". I do not know what procedure you used do accomplish this. However, a FAT format for this partition caused the Windows installer to incorrectly believe you have a MBR partitioning scheme. You should have formatted this partition as "Mac OS Extend (Journaled)".
When the Windows installer asks "Where to install Windows?", you should proceed as follows. First, delete the fourth partition. Next, select the free space. Finally, select "Next" to continue.