JNCIA study guide question - need to understand

[edit interfaces ge-0/0/1]
user@router# show
unit100 {
vlan-id 100;
familyinet {
address /24;
}
}
[edit interfaces ge-0/0/1]
user@router# commit check
[edit interfaces ge-0/0/1]
'unit 100'
Only unit0 is valid for this encapsulation
error: configuration check-out failed

Referring to the above, you are unable to commit the configuration for the ge-0/0/1 interface.

What must you do to commit the configuration?

A. You must set the vlan-tagging parameter under the [edit interfaces ge-0/0/1] hierarchy.

In the example above, the answer says we need to be in the edit interfaces ge-0/0/1 hierarchy.

I was under the impression we are in that hierarchey, from the edample.

Any help on this.