You attempted to install Office 2019 Standard (VL) on a machine that previously had Office 2019 Professional Plus (Retail). Even after uninstallation, the retains the old product code.
A Retail installation contains different and product IDs than a Volume installation. The OSPP (Office Software Protection Platform) client checks these internal IDs. If it detects a Retail product ID but the user is trying to inject a Volume license key, it throws the error: "Could not be converted." You attempted to install Office 2019 Standard (VL)
Use the Office Deployment Tool (ODT) with an XML file specifying Volume licensing: You attempted to install Office 2019 Standard (VL)