Are you sure that depth buffer support is given in this game?
Without depth buffer (the texture that stores the distance from camera to any object onscreen per pxel) MXAO cannot evaluate the occlusion between objects. Effects like lumasharpen only use the color buffer so they work without a problem. Almost every game has a depth buffer but sometimes ReShade cannot find it.. To check if missing depth buffer data is the issue, do the following: open up MXAO.fx file, scroll down to the techniques section and right above that there should be a line somewhere that says
res = color;
Right after that,, before the }, create a new line that says
Save and close.
Open up the game again and the scene should be a mixture of black and white If it is fully black or white, do check for objects that are close to camera etc, if you see any change onscreen it means that the depth buffer is recognized, just the function that prepares it for MXAO is not fitting. If it still stays entirely black or white, depth buffer is not recognized and you can't do anything about that.