Q: [Help Please] Can't import video file to iphone

Did you make sure that your video format is supported? If not, you'll have to convert it, before you can sync it to your phone.

About video formats

Using one of the devices above, you can play these video formats:

• H.264
  File formats: .m4v, .mp4, and .mov
  Video: Up to 768 kbits/sec, 320 x 240, 30 frames per second (fps), Baseline Profile up to Level 1.3.
  Audio: AAC-LC up to 160 kbits/sec, 48 Khz, and stereo audio.
• MPEG-4
  File formats: .m4v, .mp4, and .mov
  Video: Up to 2.5 Mbits/sec, 480 x 480, 30 fps, Simple Profile.
  Audio: AAC-LC up to 160 kbits/sec, 48 Khz, stereo audio.

Other formats might be supported by newer devices. Check your device's technical specifications for more information.

copied from What video content can I watch on my iPhone, iPad, iPod, computer, or Apple TV? - Apple Support

Since this is a 3rd party app, did you try to contact the developer about that error message?