This error occurs when the license is already active on another computer. You must  follow the steps below to deactivate and release the license from that PC before it can activated it on another PC.

1) In the activated machine , open DraftSight.

2) Go to Help > Deactivate DraftSight

3) You will be prompted with SOLIDWORKS Product Activation wizard.

4) Please select DraftSight > click Next and proceed with the deactivation. This will transfer out your license from that computer and you can activate this serial number in the new one.

When you reinstall DraftSight, you will need to enter your serial number and then you will be prompted to activate the license again.

If you are unable to deactivate the license on that computer for any reason, please report the problem to DraftSight Technical Support using the support email address provided on your purchase receipt. Please include your DraftSight serial number when you report the problem.