I downloaded “SILVA_138.1_LSURef_NR99_30_06_20_opt.arb.gz” from the arb-silva website to use as the classifier for 18S protozoa data in qiime2 2019.10.
The arb file is not the correct file to use. That is for use within arb. As @colinbrislawn suggests you can download the fasta, and other files directly. For this, I'd highly recommend using RESCRIPt to download the files you need. Note: due to some recent changes, you may need version qiime2-2021.2.
You can follow the approach outlined in this thread: