Hi @Crisky!
So the warning message here is just saying that you have one version of scikit-learn installed in your environment (0.22.2.post1), but you are trying to use a pre-trained classifier built using a different version of scikit-learn (0.21.2). For a variety of reasons, we disallow mixing versions like this, because it can lead to all kinds of problems.
Taking a step back - how did you get scikit-learn 0.22.2.post1 installed in your QIIME 2 conda environment? There must've been some kind of manual step in there, because QIIME 2 2020.2 currently ships with scikit-learn 0.21.2, and wouldn't be automatically upgraded unless you either tried to upgrade yourself, or maybe you tried to install some other software in the env?
The quickest way to get moving is to just create a new QIIME 2 environment, it should go pretty fast since you already have the packages downloaded and cached. Once the new env is created, verify the version of scikit-learn installed in it by running:
conda list scikit-learn
:qiime2: