You may be getting this error:
Please update to the latest Anki version, then import the .colpkg/.apkg file again.
while trying to convert apkg file and this is because apkg was exported from anki without supporting previous versions. (there is a checkbox that must be checked in anki)
Please see this video on how to export correctly and try to reexport apkg file with support of previous versions and convert new apkg file with anki apkg to pdf or other format needed.
Here is video tutorial on how to resolve this problem: