Thank you for the suggestion!
I take a look of the 7-level taxonomy of Silva, there are lots of taxonomic is missing the D_6, so I use the taxonomy all level.
However, even used the all level, I still need to use --p-confidence=disable , otherwise I still got the same error. This doesn't happen in previous version of QIIME2.
Anyway I got my result successfully!
Thank you for the help!!
Sung