Suppose I have a Sharktocrab and my opponent has a Cavalier of Thorns in play. They attack with the Cavalier, and it becomes tapped. On my turn, I Adapt my Sharktocrab, putting a +1/+1 counter on it and causing its ability to trigger.
Can I target their Cavalier with this ability, preventing it from untapping during their next untap step? Or is it an invalid target, because of rule 701.20a:
To tap a permanent, turn it sideways from an upright position. Only untapped permanents can be tapped.
Arena lets me do this, but it seems like it should be forbidden.