I get this error message when trying to use both these mods:

Found conflicting mods: requiem conflicts with {optifabric @ [*]}
Creating Empty clause ?

Anyone knows why this happens or the solution to this error?

You don't.

Optifabric is bad, always has been. Use Iris or Canvas instead. These mods conflict, that's why it won't let you start the game

Well i know that i could use Iris or Canvas, but i still want to use shaders. Is there some mod that could allow me to use shaders without optifine and on Fabric?

Well i know that i could use Iris or Canvas, but i still want to use shaders. Is there some mod that could allow me to use shaders without optifine and on Fabric?

Yes, Iris and Canvas.