I seem to have reproduced a similar error with the sample data for the shortlist template. Try adding the following webmapID to the template:
461a25dfa5484cc2affaa6445bb4cd0a
Perhaps labels are not supported because the template converts the symbology to number markers for the locations.
Is the layer being labeled a point layer which will define the tabs or is it a supporting layer?
In theReadme it specifies the following:
"In your web map you don't need to specify symbology for the point layer(s) that define your tab. The Shortlist automatically applies its own built-in numbered marker symbols to the data in the layers. So it doesn't matter what symbology you apply to these point layers in your web map: it is ignored by the Shortlist"
"Unlike with the point layer(s) that contain the places in your tabs, the symbology you choose for your clickable supporting layers in your web map is used as-is by the Shortlist. The Shortlist doesn't apply its own symbology to your supporting layers. So be sure to symbolize your supporting layers the way you want them to appear in the map. Scale visibility range settings are also taken into account."
