Only business class systems and perhaps a few other high-end PCs include access to the Advanced section.
The limitation is meant to protect against changes that can damage the computer and to protect the manufacturer (HP in this case) against false claims ("It failed for no reason").
BIOS – Backlit Keyboard and other Advanced Options
This is one of those situations where the how-to document mentions that a particular feature might not be supported on every computer. The document does not offer an exact explanation because the document is not written for YOUR computer.
Section: The back light stays on for only a few seconds
Note: If there is not a setting for the back light timing in the BIOS, this behavior might not be supported on the notebook. Check your notebook model's product support documentation to confirm support.
In general, and there might be exceptions,
Business class computers offer the BIOS Advanced section.
Examples of business class systems include and are not limited to: Zbook Mobile Workstations, ProBook, EliteBook, EliteDesk, and others.
The backlit keyboard timeout option is included in the Advanced section as appropriate.
At this writing, consumer class systems may not include the Advanced BIOS section.
Examples of consumer grade systems include and are not limited to: Spectre, Pavilion, Envy, Notebook.
BIOS configurations vary depending on the computer. For further information on a specific computer, refer to the documentation provided with the computer or go to the appropriate document for your PC.
Consumer Class Desktops and Notebooks – BIOS Setup
Be alert for scammers posting fake support phone numbers and/or email addresses on the community. If you think you have received a fake HP Support message, please report it to us by clicking on "Flag Post".