HI @gpgarcia,
This is because you are running an old version of QIIME 2! I recommend just installing the latest version if you can.
If you notice, the README on that repo notes that that has been superceded by another repository, which has been superceded by RESCRIPt.
And the great @SoilRotifer himself recently wrote a tutorial for using this new plugin to create a SILVA classifier:
Though the QIIME 2 data-resources page also hosts some pre-formatted SILVA sequences and taxonomies that you can use to train a classifier, no need to follow that whole tutorial.
But the easiest thing of all would be to install the latest version of QIIME 2 so that you can use the latest pre-trained classifiers.
Good luck!