The possible reason why you may be seeing this is that the custom extension you are using may not allow an export to excel. Have you considered using the pivot table object which comes directly out of the box with Qlik Sense (from version 2 and above)? The latest version of Qlik Sense does allow export to excel directly from table and pivot table objects.
I hope this answers your question. If so, please mark this as correct and helpful
Hi Fabio - I agree with Mark in that the extension won't necessary offer export capabilities. In 2.1 (Latest and greatest) you can do a table object export, but the pivot table object export to excel isn't there (only Image,PDF export). You can also get a PPT export from a story slide. The good news is that the pivot table object -> export to excel capability is coming.
In addition NPrinting will support Qlik Sense and offer all kinds of reporting options.