They can't attach assets that don't exist, but they can garnish your wages. The best thing to do first is speak to a tax attorney, who will likely try to make an arrangement with the city for a payment arrangement. They're usually pretty reasonable about it, when they understand that there's been an honest mistake made. They don't want to destroy you, they just want their money, and they're generally willing to work it out so they can get it in a way that doesn't destroy you.
If you're not comfortable with hiring an attorney, you can call the city yourself, if you're more comfortable with that. I've heard of several instances when the city was willing to work with a taxpayer directly.
(I'll admit a little bias, since I am myself a tax attorney)
Either way, best of luck in resolving your issue.
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