So... can I put Force Field on a figure with an SP that allows the use of Toughness, Invulnerable, or Impervious? Since I can't use them as a prereq for a feat, does that mean I can ignore them for Force Field?

No luck. The restriction is:
Powers that let you use damage reducers still reduce damage, so they are out. Had it said "Choose a character without Toughness, Invulnerability or Impervious" it would probably have been allowed, but that's not the case.

And you CAN'T use Feats on SPs that meet the Feat's pre-reqs.

Old song sung over and over again...

And I misread the original question.

That's a good "loophole" but yeah...won't work. The SPs grant you an ability to reduce damage which pretty much makes FF invalid, SP or not.

Right, that's why I was thinking maybe SPs don't apply in this case; if I have an SP that grants me Barrier, I can't use Force Field... so why should a power that grants me Toughness stop me from having it?

I guess it's a wording thing... the "without a power that prevents damage" doesn't come in the prereq section, it comes in the feat description...

You can if you also happen to have (natural) Telekinesis.
It's kinda strange, yes, but it makes it more restrictive: Had it been in the prerequisite section you would be able to assign the feat to a figure with damage reducers, but not use them when damage reducers were showing. With the current wording, you are automatically disqualified from using Force Field if you have damage reducers anywhere on the dial.