Whenever I try to open the game with both of these mods in the folder this error message pops up:

I just want to know why this happens

Thats probably because those 2 arent compatible, as i know those mods do the same so u cant run them together!!!
As it says in error message "sodium conflicts with optifabric"

From the Sodium Curseforge page: Sodium and Optifine are incompatible with one another. You must pick one.
Since Optifabric is only used to support optifine, you must be using optifine. If not, you don't need optifabric.

Duplicate of #40, as mentioned there and above it's something which will simply never work with OptiFine.