If your company has an active PTC support contract for Mathcad Prime, you are often entitled to a "legacy" license for Mathcad 15. You can generate this through the PTC customer portal using your Site ID.
For professional or corporate use, using an unlicensed version can lead to significant compliance issues during software audits. 4. The Modern Solution: Mathcad Prime mathcad 15 license file free download link
Are you trying to from version 15, or are you starting a new project from scratch? If your company has an active PTC support