I think that by default when using KML files, the tracks don't have any timestamp information.

When geotagging Viking uses tracks to work out where a timestamped image would be located.
Hence in this circumstance there is no timestamps in the track, so it can't determine where any image would be.

Thus I would recommend converting the information in a KML file into a suitable GPX file that Viking can use e.g.:

gpsbabel -i kml -f file.kml -x transform,trk=wpt -o gpx -F file.gpx

More detailed info:

Viking uses gpsbabel to open KML files, by using the default file conversion from KML to GPX.
This seems to convert KML Paths into GPX tracks.
Paths seem to just be a list of coordinates and hence have no timestamp information.
And converts KML Placemarks into GPX Waypoints (which may have timestamps).
https://developers.google.com/kml/documentation/kml_tut

Note that one can geotag against a single waypoint - this is more for a use case of having created a waypoint for the location of something, which you've taken pictures at - and doesn't care about timestamps - since you're effectively saying these photos are at this position.

Potentially Viking could attempt to interpolate against all waypoints if there is no timestamp information in tracks.
Here I think it would need to (temporarily at least) sort the waypoints by time (as waypoints don't have any guaranteed sort order) to make the interpolation method work more reliably.

Thanks very much for this information! I'm probably not clear on the terminology but I have KML files that contain time information. And exiftool will tag the images directly from the KML file, eg exiftool -geotag track.kml *.JPG works perfectly. But if I convert the file with gpsbabel as you suggest, Viking will tag it. So I think it would be very useful for it to give an error message suggesting the user convert to GPX when they try to tag using a KML file, instead of the current case where nothing happens.

I'm making progress on writing my own KML parser as part of #90 which would also address this issue; so I'm asking if it would be possible you could provide one of your KML files as an example, either attach it to here or if you prefer to email it directly to me - rw_norris at hotmail dot com - thanks.

Here is one (renamed from .kml to .txt to allow it to be attached). As before, exiftool -geotag tagged the images correctly.
BA630-20b98300.txt

Thanks for that.
It appears this source KML file only has formal timestamps against the waypoints.
It contains time span descriptions for the lines (but these don't appear to be in a defined exchange schema format).

So I don't think my KML parser will help geotagging in this specific instance, so really Viking needs to automatically try against the waypoints (as I previously mentioned). Another job to go on to my TODO list.