Erase the disk and reformat as MacOS Extended Journaled. APFS disks are not recognized when plugged into the USB port of an old Time Capsule.
Reference: https://support.apple.com/guide/aputility/share-a-usb-hard-disk-on-your-network-aprt2703/mac
Note: AirPort Utility doesn’t support formatting disks. Format the hard disk using your computer. On a Mac, format the hard disk using macOS Extended format (also called HFS+). On a Windows computer, use FAT32.
