There is nothing he can do when the situation has already arrived to this point. If Trump were in this position right now, there’s nothing he would be able to do either. Biden’s job was to make sure that things never escalated to this level at all - he already failed the moment those Russian tanks rolled across the Ukrainian border, everything he does from this point forward is at best damage control, at worst pouring more fuel on the fire.
We can talk “what would Trump do” all we want, but the reality of the matter is that during the 4 years he was in office, Russia didn’t invade Ukraine. Don’t you think Putin might’ve wanted to do something this big when he had a subservient puppet in the White House who he knew would bluster on Twitter but never retaliate in any way, given that he’s a Russian puppet who takes orders from Putin anyways?