It’s the same bug as reported before by several people: Unable to select Spell target
It’s logged in the known issues as:
It’s happened to me on 2 different occasions and both times were in delve events, which is strange. After evaluating my play speed, I don’t think I play those events any faster than normal, but the bug doesn’t seem related to specific events or game modes. Other people have reported that it happened in class event, world event and I think even explore.