Was your first troop entangled when it triggered the charm? I’m betting yes, since Orbweaver enemy. Charm effects don’t “see” units with 0 attack, they aren’t even considered valid targets for the spell. If you use a troop that randomly charms, and everyone on the enemy team has 0 attack (whether by entangle or not), you’ll get this error message. If everyone except one troop has 0 attack, only the troop that has attack will ever be the subject of the “random” charm. A troop is also not a valid target for charm if they are not adjacent to another troop. You’ll also get the error message if you try to target charm on an entangled troop (or troop with no troop adjacent to them), as in with Viper or Lust; it is basically saying “theres nothing to charm here”.
Charm didn’t always work like this (you used to be able to random charm troops that couldn’t hit anybody either by virtue of having 0 attack or nobody next to them, then it would tell you no target was available if they couldn’t hit anybody or just deal 0 damage if entangled), but its been this way for a long time (a year, maybe more?).
Also, forced transforms are not considered kills for trait trigger conditions. Stealthy has no effect on the likelihood of a unit being able to be targeted by random charm or targeted by the damage from a charm.