Swift 3.0 GM, Swift 4.0.2
If the package libpython2.7-dev is not installed, the following error is thrown when running swift:
This was on both a fresh Ubuntu server 14.04 and 16.04, with OpenSSH in addition to the default options. Solution; the installation instructions on the site should also instruct to install libpython2.7-dev.
Update: Additionally, if libcurl3 is not installed, which it isn't by default on Ubuntu Server, then a different and more cryptic error may occur: the REPL may emit "error: Couldn't lookup symbols" errors when trying to import Foundation and other libraries. Unlike the libpython error, this one gives no clue as to what is missing. (See SR-2744 for full details.)