Can I file an extension post tax day? I submitted return on April 16th. To clarify: I didn’t file ext yet. Please advise ?

No, there is no point in filing an extension past the due date of April 15th for your 2023 tax return.

if you are due a refund, not filing a timely extension should cause no problems. on the other hand, if you owe, the IRS can send you a penalty notice for late filing and late payment. a late filed extension is no extension.