How do I display the chunk borders?

I know that sometimes you need to hold down the function key (Fn on most keyboards) for the F keys to work. Try Fn+F3+G.

One of the MC devs (Dinnerbone) has an online tool that allows you to calculate chunks. https://dinnerbone.com/minecraft/tools/coordinates/

On some computers, it is just F3+G but on mine is an HP and I use fn+F3+G

Go into chat settings and disable reduced debug info.
It should re-enable the following:

• Chunk borders
• Bounding boxes
• Coordinates
• Block info
• Chunk info.
• Facing info.
• Client and server light info.
• Biome info.
• Sound info.
• Difficulty info.

And probably more.

Source

The lines of the chunk border appear virtually invisible when running shaders. Simply, turn off your shaders if you're running them

Maybe you have the lock key on. Try unlocking it by pressing fn + lock key (esc key) and then pressing fn + f + 3

Well, this can be the thing causing the issue: A broken keyboard. I used to play Minecraft on my laptop and the function keys didn't work. Make sure that your keyboard is working.