@Tomaz if your issue is solved now, please don't forget to mark your post as such!

I realised that I am able to sign the application manually using apksigner but I don't get why it doesn't work with Qt Creator. Unfortunately, without being able to sign the app within Qt Creator I cannot publish it on the store as the "Add debug server" option is always enabled if the "Sign package" option is disabled. See below.
0_1563706963222_Screenshot from 2019-07-21 11-58-10.png