I just tried to install q2-picrust2 on my qiime2 2021.4. I have followed instruction from this page: Manually install QIIME 2 plugin · picrust/picrust2 Wiki · GitHub
After reaching step for refreshing qiime2 cache, I think I got an error message. The error message as shown below:
I also had this error. Unfortunately picrust2 is only compatible with qiime2-2019.10 so you'd have to create a new environment with that version of qiime2 https://docs.qiime2.org/2019.10/install/native/
and then use the steps given from the manual