This information applies to all versions
The reason these image files cannot be used in the Nik Collection apps is that they are in black & white mode with grey color space. All images used in Nik Collection apps need to be in an RGB color space, even if they are black & white. Both sRGB and Adobe RGB are compatible. You can use a program such as Adobe Photoshop to add a supported color space to your photos in order to make them work in the Nik Collection apps.