Sorry for maybe a stupid question (I'm a beginner). But I do not understand at all why the materials of the object are not displayed in the viewport. If anything, I tried all the display mode options and turned on the “Display Texture" parameter turned on under Display Option > Texture.

Try toggling "Display materials on objects", the checkered box icon on the sidebar.

jsmack
Try toggling "Display materials on objects", the checkered box icon on the sidebar.

Oh, that was really the problem. Thanks!