This error message is displayed when two installers start up at the same time.
If you insert CD-ROM, it starts installation automatically and a folder of the CD-ROM (see the figure below) is opened at the same time.
<Using the QL-550 as an example>
This installation screen takes some time to appear or disappears temporally. During this period, if you double click SETUP.EXE on the CD-ROM folder, two installation programs will start running at the same time and cause this error.
If this error message is displayed, close all the installation screens and restart installation.
Do not double click Setup.exe during reinstallation. The installer starts running automatically. Double click Setup.exe only when the installer does not start running after a while.