Hey @hemprichbennett,
The first thing that jumped out to me (and is a message I've received multiple times in various projects myself) is this:
I believe the conclusion we reached in this post addressing this issue was that you need to ensure that the version of scikit-learn used to train and test the classifier need to be the same (though the particular QIIME2 version does not necessarily, as there can be multiple QIIME releases with the same scikit-learn versions...).
From the error message above, it's evident that the classifier file was trained using scikit-learn v 0.21.2. Can you confirm whether or not your QIIME environment is using the same version of scikit-learn?