No, you don't have to file an extension. You have until April 20, 2019 to fix or mail in a rejected return.
If you're expecting a refund on your 2018 return, there's no rush. You have until October 15 to file, either electronically or by paper. (Technically, paper-filers have 3 years from the return's due date to file and claim their refunds, but we don't recommend waiting that long.)
On the other hand, if you owe taxes, you have a 5-day grace period to either successfully e-file or paper-file your return to avoid possible penalties. April 20 is the last day of this grace period unless you're a Maine or Massachusetts resident, in which case you have until April 22.
By this cutoff date, your return must either be retransmitted (and later accepted, regardless of date) or postmarked for it to not be considered late.
Some returns keep getting rejected through no fault of the taxpayer, such as wrong data on the government's end. If you're in this boat, file a paper return, as it can take weeks, sometimes months, for the government to make the correction.
