Sometimes, Scaffold will not prompt you for a license because it's stuck in viewer mode. This can occur if there was already an installation present and the user selected use always as viewer.
For Scaffold 5/Q+S, please go to Edit > Preferences > Display Settings and click Reset Don't Show Messages. Close Scaffold and re-launch.
For Scaffold DIA, DDA, and Elements, please go to Edit > Preferences > User Interface and uncheck Always run as viewer. Close and re-launch.
Fro Scaffold PTM, please go to Edit > Preferences > User Interface and click Reset Don't Show Messages. Close and re-launch.
If these doesn't work, you'll need to completely remove Scaffold according to the article, How to Completely Remove Scaffold from Your System.