Please make sure that the format of your media file is supported in Video Editor - you can check that in the list of supported formats.
If you use SE version of Movavi Video Editor (check full program name to find out which version you use), please note that only certain formats are available in SE, Academic or Drone version are supported.
In the versions Movavi Video Editor 10 and earlier only AVI, WMV, MOV; OGG, WAV, FLAC, APE and WMA formats are supported.
In the later versions, the AAC, MKV (only H.264 with AAC codecs) support has been added.
If nothing from the above helped in solving your problem, please refer to this artcle.