Currently, SAP Fiori tools support SAP Fiori elements and SAPUI5 freestyle projects with minimum SAPUI5 versions 1.65 or higher.
In case there’s a need to test an existing projects with a lower SAPUI5 version, the following workaround can be used to preview the application:
data-sap-ui-theme="sap_fiori_3" to a theme supported by the required version, such as 1.38.50: data-sap-ui-theme="sap_bluecrystal"