Hi Matthew, thanks for replying. I saw this post after I asked this question, and I follow the solution and it seems work, now running DADA2 (so my fastq probably are demultiplexed)
Just one last question, does multiplexed means barcodes are contained in the fastq sequences? If possible, I suggest to put the definition of "multiplex sequence" on the Qiime2 "core concept" page, thank you very much!