What app? The online program can only file 2018, and it is too early to file an extension online. That will be available in March. "Too late" may be an incorrect error message but the extension still can't be filed yet.
If you are using the desktop program installed on your own computer, each year's program is specific to it's year, so if you are using Turbotax 2017, you can't file an extension any more for 2017.
You can print and mail in the extension form if you really want to file it now and forget it until later.
