If you want to use all great projects you need to download additional mod that removes requirments from them. Most of the great projects will be useless for you because they require a certain culture (state or accepted) or (state or syncretic) religion or both. There are some really nice ones that would be great to have at the same time but that would probably be too op.

Your problem as someone pointed out is that your state religion is Sunni and you want to use a great project that has required state or syncretic religon to be Orthodox. The state religion is simple and syncretic is only available to Tengri religion. Most of great projects that you can use are based around Middle East, India and Africa.

But if you are using mods you might as well install the requirments remover for great projects.