I struggled for many months to get a new version of QIIME 2 on one machine. The issue was similar to that described by @jm_s here: Qiime 2 installation failure
My solution was slightly different, so I wanted to post it, in case it benefits someone else. Here’s what I did:
conda clean --all
conda create -n qiime2-2018.8 python=3.5.5
source activate qiime2-2018.8
conda install widgetsnbextension
# edit qiime2-2018.8-py35-osx-conda.packages.yml to remove channels and dashes ("^- "), save as qiime2-2018.8-py35-osx-conda.packages.txt
conda install -c qiime2/label/r2018.8 -c qiime2 -c conda-forge -c defaults -c bioconda -c biocore --file qiime2-2018.8-py35-osx-conda.packages.txt