This is a few months old, but my search brought my here first, before finding what I needed. So here it is a complete answer.

It’s likely that the volume is unpartitioned. The link provided above is the correct resource, but you just needed to have the context that the drive is unpartitioned.

If you’d like to review the steps:

• Go to https://docs.digitalocean.com/products/volumes/how-to/increase-size/,
• Navigate to the ‘Expand the Filesystem’ section and the follow the setups under “Expand ext4 filesystems” or “Expand XFS filesystems”, depending on your chosen filesystem.

Or just do this:
// Go here and copy the name of the volume.
// It’ll be something like scsi0DOVolume_<volname>
$ cd /dev/disk/by-id
$ ls -al

// resize2fs takes care of expansion.
$ sudo resize2fs /dev/disk/by-id/scsi-0DOVolume<volname>

// Verify
$ df -h /dev/disk/by-id/scsi-0DOVolume<volname>