SkyblockAddition uses CITs (Custom Item Textures), so it only works with optifine or any other mod that lets you use CITs.
The pack deliberately contains few textures (only the most useful ones) because the idea is to keep the pack lightweight and have a compromise between performance and comfort
In order to make the pack work you will need :
Optifine : for 1.8.9
Firmament : for 1.21.5+
Note that the pack won't work if you don't install the required mods
Enjoy !