Hi @Bill_Yen,
Looks like a RAM issue to me. Have you tried the prototype Silva 138 classifiers referenced below?
They do have a smaller memory footprint. If you want to save on even more drive space and memory requirements use the classifier without the species labels. Note, you may have to retrain the classifiers yourself. In which case, make use of the provided sequence and taxonomy qza files.
Keep an 
out for updates related to this topic.
-Best wishes!