You can file your taxes and they won't be rejected just because you didn't file an extension.
If you have a balance due you will be subject to penalties and interest on the balance due.
Here are a few links with more details
https://ttlc.intuit.com/community/file-with-turbotax/discussion/extension/01/3390239#M33968
https://turbotax.intuit.com/tax-tips/tax-extensions/filing-an-income-tax-extension/L5GIzgJaS
https://turbotax.intuit.com/tax-tips/irs-letters-and-notices/guide-to-irs-tax-penalties-how-to-avoid...
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