Hi Francisco,
FYI: An ArcGIS Online public account does not have access to Web AppBuilder for ArcGIS.
We checked and it looks like you originally started using ArcGIS Online with a 60-day trial account - which does include access to Web AppBuilder. However, the 60-day time period expired. Once the trial period expires, your trial account reverts to a public account. Which is why you can no longer access Web AppBuilder.
Hope this helps,