Avoid using USB 3.0 (blue) ports; are much more stable for bootloader exploits. Use an original Apple Lightning cable if possible. A Note on "REPACKS"
Ensure you have downloaded the iPhone 6 ramdisk files via the "Download" button within the tool's configuration settings. Check Driver Status: --- Loader For Iphone7 2-n61ap Not Found Unlocktool REPACK
If your tool is specifically asking for an n61ap loader, it is looking for files. For iPhone 6 (n61ap) : Use the A8 Ramdisk files. Avoid using USB 3
If none of the above solutions work, try using a different version of UnlockTool, specifically the REPACK version: Avoid using USB 3.0 (blue) ports