The error states that it's looking for an assocation named NameSurname, but it can't find it (because it's actually an attribute I assume)
However, the issue probably is that you can't use a Calculated Attribute for this, since these attributes aren't saved in the database, and thus can't be used with XPath.
The widget can use search XPath query on calculated attribute, as the attribuet is not storred in the DB. However i noticed the widget also support search type Microflow, you could give this a try, as the calculated values will done server side and send to the server!
Thank you for your answers, and Paul, thanks for the hint! I was already suspecting this.
I did a workaround, using a microflow to fill the contents of a data view whilst also copying the values into a stored attribute which allowed me to use the autocomplete widget with XPath again.
Thanks for your suggestion Andries. I will have a look into it for future use.