It is terrible shortcoming of the current version that you can turn the Section-Naming feature OFF, but not back ON !
It should be an option in Preferences… But it’s not.
BUT there is a convoluted way to fix it - at least on Windows…
Close SketchUp.
Find the folder:
C:\Users\YOURUSERNAME\AppData\Roaming\SketchUp\SketchUp 2018\SketchUp
It is hidden by default, but changing Folder>Options to show hidden file, or copy/pasting the [edited] folder-path into a Windows Explorer window address-bar should take you there…
Then find the file named:
SharedPreferences.json
Edit it with a plain-text editor - like Notepad++.
Find the entry that reads:
“ShowSectionToolDlg”: false,
Change it to read:
“ShowSectionToolDlg”: true,
Save the file.
Restart SketchUp…
The Section-Naming dialog should be restored !
Be careful not to change anything else !
If in doubt - stop.