Creating a map works similar to creating a regular chart inside Datawrapper. First, click here to create your chart. Your dataset has to include a column with the ID-Code of the municipalities in order for the map to render. To get started, you can select one of the example datasets Datawrapper provides for Swiss Maps.
In step 2, you need to set the Column Type of the column that contains the ID-Code from Number to Text. Your map will only render correctly when Datawrapper interprets the column as a text-field. You can change it by clicking on the column header and setting the Column Type to Text: