For those who may be missing their free copy of Amplitude or have an invalid code, Harmonix is asking you to search spam and junk folders for an email from email@example.com. For European users it may be worse due to Harmonix having trouble with the console submission process. They are currently working with Sony Europe in order to fix these issues.
The list of countries affected by these issues is as follows: Australia, New Zealand, Austria, Belgium, Denmark, Finland, Luxembourg, Netherlands, Norway, Poland, Portugal, Spain, Sweden, and Switzerland. In Australia and New Zealand the game is live in stores but codes are “currently inactive.”
However, if you are in North America and still having issues, contact Harmonix via their Kickstarter page for Amplitude. Currently the game is only available on Playstation 4 though will be coming to PlayStation 3 sometime later this spring.