Merge unallocated space in a partition (NTFS)

[Corss]

Hi, I have a huge problem. When I first installed High Sierra yesterday, I chose to partition my 1TB hard drive in two equal partitions (both formatted in Mac OS Extended (Journaled)) because I wanted to install Windows in dual boot. Today, I tried to install Windows, but when I first tried to do it, it told me that my partition wasn't formatted in NTFS and I could not do it. But now, when I open "Disk Utility", I cannot see my partition, it is like unallocated space and I do not know how to create a partition out of it formatted in NTFS. I really have to install Windows on this PC, how can I solve this?

Sorry for my English.

Edit: when I select the hard drive from the left pane, it shows me that "High Sierra" is 920 GB, not only 460 GB. Is it a bug?

 

[Going Bald]

See the guide for installing Mac OS/Win10 on same drive pinned at the start of this forum.