Hi. Thanks for posting your question in GeoNet.
This error message says that one expression in your XLSForm is referencing a question that cannot be found.
The default XLSForm templates in Connect include a couple of questions (example1 and example2) and also an expression that uses them. The expression is in the settings worksheet and defines the instance name of your form. If you delete or rename of the questions used in the expression, you get that error.
This error is not indicative of a problem with Survey123 Connect. This error is indicative of a syntax problem with the XLSForm you are authoring.
To eliminate the error: In your XLSForm, go to the settings worksheet and eliminate the expression in the instance_name column. Save your XLSForm.
You can learn more about the instance name here:
We also have a number of XLSForm tutorials to get you started in YouTube: ArcGIS Survey123: XLSForm Tutorial 1 of 5 Creating and Publishing Surveys - YouTube
For the fastest response to Survey123 questions, posting in the ArcGIS Survey123 group is the best.
Hope this helps.