原文Lockjaw's last ability will check as your beginning of combat step starts to see if you've cast a noncreature spell this turn. If not, the ability won't trigger at all. You won't be able to cast a noncreature spell during your beginning of combat step in time to have the ability trigger.
原文You don't choose a target for Lockjaw's last ability at the time it triggers. Rather, a second "reflexive" ability triggers when you put a +1/+1 counter on Lockjaw this way. If you choose a target for that ability, you do so as it goes on the stack. Each player may respond to this triggered ability as normal.
原文You don't have to choose a target for the reflexive triggered ability created by Lockjaw's last ability, but if you do, and that target is illegal as that ability tries to resolve, it won't resolve and none of its effects will happen. Neither Lockjaw nor that creature will become unblockable.