Error : File to import not found or unreadable: node_modules/office-ui-fabric-core/dist/sass/variables/Font.Variables.
Your web part.module.scss has import statement i.e. @import '~@microsoft/sp-office-ui-fabric-core/dist/sass/SPFabricCore.scss'; By default this package has not been installed. So you have to install this package : npm install @microsoft/sp-office-ui-fabric-core –save Then your import statement will work.