silva138_V3v4/V4V5_human-stool_classifier.qza

I assume you pulled human-stool.qza from readytowear?

As for the

This appears to be an unresolved issue as outlined in this thread. I would suggest either using the human-stool.qza full-length class weights or use your newly extracted V3V4 region w/o the class weights. Perhaps @BenKaehler might have some other advice here?

-Mike