According to https://jira.mongodb.org/browse/COMPASS-3705 and I experience the same, the drag functionality for re-ordering stages in the Aggregation Builder is broken and fixed only in 1.21.0.
This version isn’t offered on the download page – is there a source for Compass with this issue fixed?
FYI, Compass 1.21.0 is now released and available via auto-update within Compass (you should be prompted automatically) or from the MongoDB Download Centre.