Hi Sergio,
The location button in the Geoform app (and other apps) uses the Geolocation API to prompt the user for permission and find their position. Starting with Google Chrome version 50 and potentially other browsers, some HTML5 web APIs will require using a secure origin like https to work correctly such as the Geolocation API.
This blog explains this in further detail: Increased web API security in Google Chrome | ArcGIS Blog
As a result of these changing standards, the locate button shouldn't be displayed for apps that are loaded via http. There is a bug where the locate button appears with some mobile browsers that is resulting in the error that you are seeing when you try to use the locate button with an app that loads of http. The bug is being worked on for a future release.
Long story short: If you want to use the locate button, ensure that you load the app over https.
-Kelly