Empower Jace won't let you put loyalty counters on nontoken Jace planeswalkers. However, if you control a Jace planeswalker token that you got another way (such as an effect that creates a token that's a copy of a permanent), empower Jace will put loyalty counters on that.

Excess damage has been dealt to a creature if the damage dealt to it is greater than lethal damage. Usually, this means damage greater than its toughness, although damage already marked on the creature is taken into account.

For a planeswalker, excess damage means damage in excess of the planeswalker's loyalty (the number of loyalty counters it has on it).

If you control a Jace planeswalker token, you can't choose to create a new one when you empower Jace. You have to put the loyalty counters on your existing Jace planeswalker token. In the rare case that you control multiple Jace planeswalker tokens, you'll choose one of those tokens to put the loyalty counters on.

Some spells and abilities that empower Jace may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't empower Jace.

To empower Jace N, if you don't control a Jace planeswalker token, create a blue Jace planeswalker token with 0 loyalty, "[−1]: Surveil 1," and "[−3]: Draw a card." Choose a Jace planeswalker token you control. Put N loyalty counters on it.