PRD wrote:It don't say: "You could try" or "in wick you have trained", it say "you can't achieve alone". So if you are unable to get a success with a roll of 20 on the dice, you can't help.
In cases where the skill restricts who can achieve certain results, such as trying to open a lock using Disable Device, you can't aid another to grant a bonus to a task that your character couldn't achieve alone. The GM might impose further restrictions to aiding another on a case-by-case basis as well.
It's a matter of where you put the emphasis. The listed example is Disable Device, a trained only skill. If you didn't have ranks in Disable Device, you can't use it. I believe that's what the line means: you can't aid on skills you can't try yourself.