Online extensions have been shut down. The only way to file an extension now is on paper and mail it (postmarked before June 15 midnight - best to mail certified so you have proof of mailing.)
https://www.irs.gov/pub/irs-pdf/f4868.pdf
Note: if you are due a refund then there is no penalty for filing late.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**