The software is the software, is it a student license? And was a student license previously installed on the workstation AND are you opening a file that was created or saved in a student version of the software?
If you previously had a student version, uninstalling it is not enough. You may have to remove all the AutoCAD stuff from the registry too (I'm assuming Windows here). If you're not a registry-editing sort, CCleaner will do it after the uninstall. If not, RegHunter ($39) will definitely do it.
Best to you,
Goober Dave
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The software is the software is the software (not the License).
Uninstalling, fooling with the Registry ... will not change the License (only the software).
To change the License - open the software an go to Help>About AutoCAD>Product Information and Click Update for the Serial Number. Enter a valid commercial serial number to replace the student number and follow the on-screen instructions to Activate the new License. (Any student license can be converted to a commercial license in this same way - no need to uninstall/re-install.) Of course you need to purchase a commercial license (serial number) and any work created previous to the commercial license would be appropriate only for the terms of license of a student license.
If you do not have a valid commercial license Serial Number - contact your VAR.