If you enabled the pack but don't see any light numbers on blocks, work through these checks. They use only the standard resource pack menu.
Check the pack is actually active
1. Open Options → Resource Packs.
2. Confirm 0light1levels15 Shaders is in the Selected list, not Available.
3. If it shows a warning that it was made for a different game version, that may be the cause. Core-shaders depend on the game's built-in shaders.
4. Click Done and let resources fully reload before you check again.
Check for conflicting packs
1. Look at the other packs in your Selected list. Packs higher in the list override packs below them.
2. Another pack that changes shaders or rendering can replace this pack's effect. Move 0light1levels15 Shaders to the top of the list.
3. If the numbers still don't show, temporarily deselect the other packs one at a time to find the conflict.
Test in a clear spot
1. Stand right next to a torch and look at the nearby blocks. They should show the brightest reading.
2. Step into a fully unlit area to check for the darkest reading.
This pack is a core-shader for standard Minecraft and doesn't need a shader loader. If you also use one, try with it turned off to rule it out.