What do I do if the irs tried to withdraw what I owed and I didn't have enough funds in my bank account at the time?

IRS website for payments made by electronic transfer - https://www.irs.gov/payments/pay-taxes-by-electronic-funds-withdrawal

If a payment is returned by your financial institution (e.g., due to insufficient funds, incorrect account information, closed account, etc.) the IRS will mail a Letter 4870 to the address we have on file for you, explaining why the payment could not be processed, and providing alternate payment options.

In the event your financial institution is unable to process your payment request, you will be responsible for making other payment arrangements, and for any penalties and interest incurred.

Go to this IRS payment website for your payment options - https://www.irs.gov/payments

The IRS will send a bill for the tax owed, plus a bounced-check fee, and possibly interest and a late payment penalty.