Hi @Jia,
The error seems to indicate there were no matches based on those primers on the ref database you provided. Which greengenes file (99_otus.qza) exactly are you using? By chance you used the aligned rep_set one by mistake?
You are right. I used the 99_otus.qza generated from the fasta file from the aligned rep_set folder by mistake. I tried the other rep_set folder and it’s working now.