If you have two (or more) different python installations on your computer edit the openage.bat in the install directory:
Replace the line python.exe -m openage with call "%INST_DIR%\python\python.exe" -m openage to start python.exe explicitly.
Check if you have passed the root folder of the game to the converter and not a subfolder.
If that doesn't help, you could have a mod installed that messes with the detection algorithm. In that case, you should reinstall a clean unmodded version of the game and retry the conversion.
Make sure you don't have UserPatch, compatibility patches or modifications installed that make changes to the original asset files.
If you have Wololo Kingdoms and various mods installed that change the base assets the converter will not work.
A workaround would be to make a backup of your AGE2 directory and let the converter run on that backup. In that
backup at subfolder AGE2/resources delete all files except folders. Another workaround would be to
backup your AGE2 folder and redownload it to have a clean install. After conversion you can replace
it with the backup.