If you genuinely cannot pay, the IRS has a formal status for that: Currently Not Collectible. When you qualify, the IRS stops all collection activity. No levies, no garnishments, no phone calls. And the 10-year collection clock keeps running.
If you can pay something but not the full amount, you may qualify for an Offer in Compromise (settling for less) or a partial-pay installment agreement (monthly payments that don't cover the full balance before the statute expires).
The worst thing to do is nothing. Penalties add 25% or more to your balance. Interest compounds. A $20,000 problem becomes a $50,000 problem if you wait long enough.