The IRS sent a bill for 2018. They claim we didn't report all the income we received. How do I find out where that claimed income is from?

[xmasbaby0]

You can get a wage and income transcript that will show all the income reported to the IRS for you:

https://www.irs.gov/individuals/transcript-types-and-ways-to-order-them

TurboTax does not receive any information from the IRS or your state after your return was filed, so no one at TurboTax knows about a letter you received from the IRS or the state.

(what the letters from the IRS mean)

I GOT A LETTER FROM THE IRS   

https://ttlc.intuit.com/questions/3605673-i-got-a-letter-or-notice-from-the-irs

I AM BEING AUDITED

If you purchased Audit Defense:

https://ttlc.intuit.com/questions/2924451-i-bought-max-or-audit-defense-and-now-i-m-being-audited

AUDIT SUPPORT

TurboTax has a special phone number for help with IRS/state tax letters, which will be easier than going through regular Customer Support.   At the page below, click on the blue button that says "Get Help from TurboTax Support".  Then it will ask you what tax year is your letter.  Then it will ask you what the letter concerns. Then it will show you the phone number during posted business hours.

https://support.turbotax.intuit.com/irs-notice/audit-support/

[Critter-3]

Are you looking at a CP2000 notice ?   If so keep reading carefully  ... the missing income should be listed.