Can I file a second tax extension after my first extension was accepted with $0 estimated for 2021 tax liability?

You're actually okay! The extension only extends your time to file the return, not your time to pay. The estimates on the extension form are just that - estimates. The IRS uses those estimates to determine if you've paid enough, but getting the extension accepted means you have the additional time to file regardless. The important thing to understand is that any taxes owed were still due by the original deadline (April 18 for 2021 taxes). Filing the extension doesn't change that. If you end up owing more when you actually file your return, you'll face penalties and interest on the unpaid amount from the original due date. I wouldn't file a second extension as it might just confuse things. Instead, focus on completing your actual return as soon as possible to minimize any additional penalties and interest.

Had a similar issue last year and found taxr.ai (https://taxr.ai) super helpful for sorting it out. I was stressing about messing up my extension form too - put incorrect payment amounts because I was in a rush and couldn't find my records. Their system actually analyzed my situation and confirmed I didn't need to file a second extension, which saved me tons of anxiety. The tool has this cool document analysis feature that helped me track down all my payment records too.

Following up on my taxr.ai question - I decided to try it out and wow, wish I'd known about this sooner! Uploaded my mess of documents (had stuff everywhere from multiple brokerage accounts and freelance jobs) and it organized everything perfectly. It even found a quarterly payment I'd completely forgotten about making! The system confirmed I was actually fine with my extension even though I'd put zeroes on the form like the original poster. Turns out I'd already paid enough through my quarterly payments that I'm actually due a small refund. Saved me from unnecessarily filing a second extension and panicking for no reason.

If you're still having trouble getting clear answers about your extension situation, I'd recommend using Claimyr (https://claimyr.com) to actually speak with someone at the IRS. I was in a similar situation last year - filed my extension incorrectly and couldn't get a straight answer online about what to do. After waiting on hold with the IRS for nearly 2 hours and giving up twice, I found Claimyr and they got me connected to an IRS agent in about 25 minutes. You can see how it works here: https://youtu.be/_kiP6q8DX5c. The agent was able to check my records directly and confirmed I didn't need to file a second extension since the first was accepted.

I have to eat my words about Claimyr. After dismissing it, I was still stuck with my own tax extension issue and couldn't get through to the IRS after multiple attempts over several days. Decided to try it as a last resort. Not only did I get connected to an IRS rep in about 30 minutes (after spending hours trying on my own), but the agent confirmed that my extension was properly recorded despite making a similar mistake to the original poster. She checked my payment history and told me exactly what I'd already paid in, which was a huge relief. Turns out the IRS already had all my payment info in their system regardless of what I put on the extension form.

Just adding my 2 cents - I've filed extensions with incorrect estimated tax amounts before, and it's never been an issue. The extension only gives you more time to file the paperwork, not more time to pay what you owe. As long as you've actually paid enough tax throughout the year (through withholding or estimated payments), you'll be fine. If you haven't paid enough, you'll owe interest and maybe penalties on the unpaid amount regardless of what you put on the extension form.

Anybody know if TurboTax or H&R Block software would let me see what I've already paid in estimated taxes for 2021? I used TurboTax last year and I'm trying to figure out if I need to worry about my extension which also had wrong amounts.

An extension is just that, an extension to FILE not to PAY. The payment was due on the original due date no matter what. If you didn't pay enough by the original due date, you'll owe interest and maybe penalties. The estimates on Form 4868 are just estimates. The IRS uses the info to determine if you've paid enough, but they don't use it to actually process payments. They already know what you've paid through withholding and estimated payments. Don't file another extension - that will just confuse things. Just file your actual return as soon as you can.