Can't reinstall Solaris 11.3 - no more disk slots available

I tried to reinstall GRUB2 on my previous Solaris 11.3 installation but could not figure out why "bootadm install-bootloader" failed. Now I am trying to reinstall Solaris 11.3 instead, over my old Solaris partition. I am using the USB Live media ISO with the GUI. Installation fails with this message:

"The selected disk has no more available slots to create a system boot partition.
Allow at least one unused partition"

I have since earlier partitioned my system disk into Solaris 11.3 and Windows10. First I installed Windows10, and then Solaris 11.3. I am trying to install Solaris 11.3 and I can choose to install Solaris onto these partitions:

c3t2d0p1    330 GB     -       Solaris main partition
c3t2d0p2    256 MB     -      EFI system partition
c3t2d0p3    16 MB       -      MS reserved, scratch area
c3t2d0p4    492.32GB  -       Win10
c3t2d0p5    852 MB     -       Win10 recovery partition
unallocated    1 MB     - unallocated
c3t2d0p6    108.09GB  -       TemporaryNTFSPartition
unallocated    1.62 MB - unallocated
c3t2d0p7    8 MB        -       Solaris help partition

So I select the first partition, "Solaris" and click "Next" but then I see this error message above, "no more available slots, yadda yadda". How should I proceed? Can I mark "EFI system partition" as "unused" which means I have two available slots? But is Win10 using "EFI system partition"? Or is Solaris using it? I am afraid to bork my Win10 installation as I try to free another slot. Any advice?