Your WordPress versioning seems to be corrupted, since it’s only displaying a dash right after the version info. In cases where the WordPress auto updater isn’t properly working, i would recommend to manually re-install / update WordPress via FTP. You have two possibilities:
You manually download your current WP version from the official WP download page and replace the existing ones on the server with downloaded ones. This may fix your corrupted files and you can then do the core upgrade to version 5.
You manually download the latest WP version from the official WP download page and replace the existing ones on the server with downloaded ones. (This may break things, because it’s a major upgrade from 4.x to 5.x)
Whatever way you choose, before starting the manual installation, you need to make a backup of your database (e.g. with PHPMyAdmin)! Better, create a full copy of your current installation via FTP.
