Phone can't access internal storage

I have a problem with my relative's phone. He has a ZTE Blade A452 phone with Android, and it seemingly can't access its internal storage at all. There is constantly an error message "Your internal memory is full", and "Email service has been stopped" because of lack of storage (even though he doesn't use it at all), no new text messages could reach his phone (I switched the text message storage to SD card). When I go to Storage option in Settings, under selecting the default location there is only SD card option, and when I scroll down to phone storage to see how much is used, it says "Calculating..." and never shows the storage. It also shows some weird number in space available. After I removed the SD card, it shows there is around 4 GB of system storage, and 4 GB of user storage, yet none of it is available. Also, when I go to the file manager, I can only see the SD card and no internal storage. I can't even transfer photos from internal to SD card, because I don't see them in the manager. Here are photos: Is the phone completely screwed up or can I do something about this?