Good morning! Thank you all for your availability and your help! Unfortunately a lot of advice, even online advice on other sites, is too technical for me, who just started with Unreal.
In the past I had managed to export a VR demo with UE 5.1, so I drastically decided to delete everything: Android Studio, Visual Studio, Unreal and all the folders created in the pc. I re-installed everything, following the documentation for the UE 5.1 version, and by doing so I was able to export a working .apk to Meta Quest without any problems!
Now I have to figure out if I can convert the project from UE 5.3 to UE 5.1.
Thank you very much for your helpfulness
Deborah