I was told that according to the Indian Act - a non-native cannot sue a native. Is this true?

Sue for what ?
What is your cause of action ?

They borrowed money from me. I had a promissory note - but they refuse to pay me back. I contacted their employer - and they said a non-native cannot garnish a native.

Are you in the U.S.

Tribes frequenty have quasi-sovereign immunity. Is it a tribe that borrowed the money, or an individual?

What jurisdiction (state, assuming you are in the US) is this? Is the native on a reservation or working for a tribe?

I don't believe there is anything called "the Indian Act". So it's possible whoever you are talking to is at least partly mistaken.

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I am in Canada. It was an individual that borrowed the money. He is working at a native company.

They borrowed money from me. I had a promissory note - but they refuse to pay me back. I contacted their employer - and they said a non-native cannot garnish a native.

That is very different from suing them. I'm not familiar with the Canadian Indian Act. But I would suspect that you were told the truth. You can sue and probably win, since you have a promissory note.

However, if he is employed by the tribe, that employer is probably not subject to Canadian civil law. So they can refuse to honor a civil judgement.

If that happens, I would contact tribal authorities and ask how you can file suit against him. Indian authorities like their autonomy but they don't want tribal members to use it to cheat people. So its possible the tribal authorities will allow you to sue in a tribal court.