I can't highlight the inner faces after using the "Solidify" modifier. What to do? Blender

You need to apply the modifier first. All geometry within your modifier stack is procedural, and cannot be edited before applying. You first have to enter ObjectMode and then, in the modifiers tab, click the arrow icon on the top right corner of your Solidify modifier and select Apply from the drop-down menu. After this you should be able to select the newly created faces in EditMode