That is a known problem. See this TurboTax support FAQ - https://ttlc.intuit.com/questions/4603666-why-does-turbotax-online-say-the-deadline-for-filing-an-ex...
TT has not started 2018 extensions yet. You are seeing 2017 stuff. Do the IRS paper form now https://www.irs.gov/pub/irs-pdf/f4868.pdf