SCP: raster is not loaded

I have installed the semi-automatic classification plugin (SCP v. 7.7.1) on QGIS 3.4.11 (Madeira) using Kubuntu 18.04.

I downloaded Sentinel 2 (Level 2A) images and wanted to test one of the semi-automatic classification procedures. However, I failed to create a training input file (*.scp) getting the following information on my QGIS main window:

Information [4]: Select a raster; raster is not loaded

Furthermore, the following information is registered in the log file:

I have the suspicion that there is something not working between QGIS and other libraries such as GDAL. So, how can I inspect the issues more thoroughly and how can I resolve it?

sudo apt install qgis*