Not directly. The TravelTime add-in runs in ArcGIS Pro, and there’s no TravelTime tool inside ArcGIS Online.
You can still get TravelTime results into ArcGIS Online:
Publish from ArcGIS Pro: run the TravelTime tools in Pro, then share the output layers to ArcGIS Online as you would any other layer.
Use the API: if you build your own apps, call the TravelTime API directly.